Generally the articulations that occur when pressing SW1 or SW2 (hold down one of the switches then play a note) - these would trigger certain elements using XA control.

What you would do would be to turn off XA control on the elements.  Then edit then velocity range for each elements.  All elements that are not the articulation sound change the maximum velocity to some high level less than maximum.  Like say 120.  Then the articulation sound set the velocity minimum to 121 (one plus all other element max) and max for this element to 127 (maximum).

It's a lot to do but now high velocities trigger one element and all other velocities trigger the others.

Send CC 86 (Assign Sw1) or CC 87 (Assign Sw2) to the Modx from an external device.

Im using MODX plugin 99% of the time with external controller often.

What 'MODX plugin' are you referring to? We aren't aware of any.

unfortunately most articulations are triggered by a switch instead of velocity.

How an articulation is triggered depends on the articulation and how it was encoded. Some AWM2 samples have velocity-encoded articulations but Yamaha can also use other triggers in their samples/waveforms that users don't have direct access to.

what's the easiest way to trigger those articulations with a midi keyboard?

The assign switches themselves can be 'triggered' using their assigned control number. See p.41 of the operation manual.

https://data.yamaha.com/files/download/other_assets/2/2172442/MONTAGE-M_operation_manual_En_v300_K0.pdf

As the graphic shows the default CCs are 86 and 87 but you can change them.

for example nylon guitar slide-in articulation.

That performance uses assign switch 2 with a destination of Var Send.

My interpretation is that the external keyboard can't send the CC values that would map SW1/2 onto buttons on the external MIDI keyboard.  And this is what led to the question if how to refactor the Performance to use velocity sensitive elements that act like other velocity articulations like the guitar that does slides when you hit a key with high velocity.

So, yes, you could make your MIDI keyboard send the CC values that map to SW1 and SW2 if that's within the capabilities of your keyboard.  And also assuming you even wanted that even it could be done that way.  You might not like this way of triggering articulations and that's the driving motivation.

Otherwise, you'll need to do a series of Performance edits to remove XA control and instead use element velocity ranges to mimic what the nylon guitar Performance is doing.

More info would help - there are several 'nylon' guitar perfs and some don't use XA, some have velocity ranged elements and some use the assign switches to switch elements but without using XA.

Maybe OP will provide a specific example.

They made it clear they don't want to use switches (buttons - SW1, SW2) and do want to use velocity to trigger a different articulation like any nylon string performance that works this way.

The information was fairly complete.  I agree to be absolutely explicit then absolute Performance names that match the data list Performance names (presets) with 100% match to every character then this would remove any chance for ambiguity.

That said, the very succinct original post already clearly stated what the goal is and also tells everyone the exact type of Performance that is being alluded to here.

@Jason - glad you have all the info you need - but the suggestions you provide are likely too generic in nature to be of much help if OP is really looking for a solution to a specific problem.

They made it clear they don't want to use switches (buttons - SW1, SW2)

Hmmm - clearly we interpreted things differently

unfortunately most articulations are triggered by a switch instead of velocity.

We don't agree with that statement but didn't think it useful to argue the point. Instead we read it in the context of the next two statements:

what's the easiest way to trigger those articulations with a midi keyboard?

for example nylon guitar slide-in articulation.

The 'Nylon Slide Vel' performance is the only one we could find that uses assign switches for 'articulations'. Element 2 uses the 'Nylon 1 Slide St' waveform but it uses it when both switches are OFF.

So the articulation is NOT triggered by turning a switch on but by turning a switch OFF.

Hence we ask OP to provide the specifics for their use case. Our thinking is that the more info we have and the more specific the info is the easier it becomes to provide possible solutions to a specific problem. We're surprised you don't agree with that assessment.

The 'slide' articulations we are familiar with use a waveform with the word 'Slide' in it: Nylon Slide St, Nylon Slide L, Nylon Slide R, Nylon 2 Slide, Flamenco Slide Up, Flamenco Slide Down, Steel Slide St and others.

And the performances we checked with 'Slide' in their name that use the 'Slide' waveforms use the slide element when the switches are OFF but use it for high velocities: 121-127.

So a user can already control whether the 'slide' will occur using velocity alone. You only need to disable the slide using a switch if you need to play at very high velocities without the slide occuring.

The default 'slide' implementation is, thus, somewhat backward from other uses of the assign switches as OPs working suggests:

unfortunately most articulations are triggered by a switch instead of velocity.

 Articulations triggered by turning a switch ON, in our experience, tend to be those that use Yamaha's embedded articulation functionality that isn't available to the user. In particular their 'MEGA' voices fall into that category.

the very succinct original post already clearly stated what the goal is and also tells everyone the exact type of Performance that is being alluded to here.

Sorry - but we don't agree with that at all. And if YOU have all the info you need then it doesn't really contribute anything towards helping OP to suggest that there is no need for them to provide additional info. Doing that is counter-productive in our view.

Let's leave it to OP to decide how much, and what, info they want to provide in order to get help solving their problem.