If, as frequently happens, you attended undergraduate school at one place and took graduate courses elsewhere, the "common feeling" of most of those of my acquaintance is that the original school, where you did your undergraduate attendance invariably is considered, forever, your school. The only accepted excuse for changing your "alumni association" is if your graduate school looks much more impressive on your resume, although allowance is made for migration to faculty at a school you attended later in your academic career.

This probably is largely due to the "social activities" typically associated with undergraduate attendance, which frequently are less a part of a graduate studies since you actually are expected to study then.

If you spent less than the standard full-course four years at a school as an undergrad, many/some alumni may well sneer at you as a "short-timer" regardless of which school you choose to claim, so you may take your pick.

If your undergrad studies were split between schools, and neither school makes you "feel part of it" to the extent that your allegiance is "required," then it's probably immaterial whether you make real claims on either. Put your work experience on the resume.

Or it may just depend on which friends you go to the game with.

John