With the SRB-A part, I cannot change segment count in Sandbox.
After cheating in completed techs in Career and Science mode, it still does not apply the SRB segment upgrades.
Do note, I have checked to ensure part upgrades are enabled in the Game Settings menu.

[v1.3.1 fresh install with SSTU version .144]

Have to checked the box in sandbox settings to apply all part upgrades?

@T10a would you mind uploadind a KSP.log file, as well as your save/persistent file?

The log will tell me if there is anything interfering or any plugin-side errors, and the save file will tell me the current purchased state of upgrades as well as if upgrades are enabled/disabled in the save.

Weird. After testing out the game again, the SRB upgrades seem to be fine in sandbox & career modes.
Will do further testing to find which mods conflict to cause the issue.

After further tests, I've found it is SSTU Nova addon doing this (it adds SRB diameter upgrades, which conflict with the new SRB changes). Issue closed, and will notify JoseEduardo.

Thanks for investigating the problem a bit yourself :)

Good thing is I just saw Jose around the forums recently, so he should be back from his ARK induced KSP break. If you can't get ahold of him otherwise, please let me know -- I've got some other contact info, and/or I might be able to figure out the fixes myself.

I left a comment on the Nova forum page, which should let him know as well as anyone else who may be having this issue.

The expansion pack breaks more things as it's not been updated since 1.2.2 I think.