Is there any difference between a .X7L file and a .X7U file, except the extension?

A .X7L or Library File will (potentially) contain 640 Performances, 2048 Waveform, 2048 Live Set slots, 256 Arpeggios, 256 Motion Sequences, 32 Curves, and 8 MicroTunings... The file can be created by assembling the above data in your USER BANK, then by setting the Store/Save “Content Type” = Library your User Data is “burned” into Flash memory (Read Only Memory). While your data is assemble in the USER BANK it is the act of “saving as...” Library will not only place the “L” extension, but significantly, it causes the MONTAGE to write any User Bank AWM2 Waveforms into Flash... it creates a duplicate in the Library area; while doing so it will repoint every AWM2 Element, in each and every Part that uses a User Waveform to this new Library installed location... which is cataloged in the File.

So User memory is RAM (Random Access Memory) you can edit and make changes to data while in User memory
A Library is ROM (Read Only Memory) you cannot make any permanent changes to READ ONLY MEMORY - as the name implies, it can only be read... Same as the Preset Waveform Data, you can’t edit the factory CFX Piano Waveform Data, it’s in the Preset ROM.

In order to make a change to Library data (be it the Waveform, or any of the other installed data) you must use “Library Import” which allows you to move and “repoint” the Waveform data to the User Bank. Then while in RAM you can edit it and then create a NEW Library.

When you Save as... “Content Type” = USER File, the data (Perf, Waveforms, Live Sets, Motion Seqs, Curves, MicroTunings), is written as is from the User Bank to the File. Additionally, your UTILITY mode settings and your custom QUICK SETUP templates are included in the User File.

Problem 1
When I delete (Utility - Content - Performances) a couple performances (with pointers to user waveforms) from User area and I save what leaves behind to a user file, there are no waveforms saved to that file, even if there are performances left who point to user waveforms!

Problem 2
When I do a Library Import of a couple of performances with 'library waveforms' the copied performances in User area still points to the waveforms in the library.

So I was wondering; is it bad practice to rename .X7L to .X7U (and vice versa) on a pc?

Obviously.

I think I understand the best practice but wanted to see if I got it. My Library is full (loving the options I found), and I have build up say a 100 user performances built off of those lib and preset performances, parts, waves. Anticipating the next update wanted to run by the process options.
1) Save/export the library and user files and save on stick. 2) Update to Version next x.x (when it happens).

Could I then load in all of my previous libraries, and User file? and would the saved user performances know where to get waves from the libs after the update??

or would I have to import the new library file and replace one of my "older" libs with my new Library, as we only have 8 slots??

I think I understand the best practice but wanted to see if I got it. My Library is full (loving the options I found), and I have build up say a 100 user performances built off of those lib and preset performances, parts, waves. Anticipating the next update wanted to run by the process options.
1) Save/export the library and user files and save on stick. 2) Update to Version next x.x (when it happens).

Could I then load in all of my previous libraries, and User file? and would the saved user performances know where to get waves from the libs after the update??

or would I have to import the new library file and replace one of my "older" libs with my new Library, as we only have 8 slots??

Assuming you are currently at firmware 2.00 you would make a BACKUP File (extension is .X7A) this backs up all 8 Libraries, your current User and all Songs. A BACKUP is literally, a complete backup of your MONTAGE as it is when you make the file.

...”only 8 slots”
That’s 5760 Performances! Only!