This is a very vague description:
she has noticed she has many more contacts than before. Some she does not think belong to her.
Contact assignment to specific GM users is NOT an "out of the box" reality. Rather, that is a very custom thing.
What determines that a contact "belongs" to someone? Is it coding in a specific field?
An upgrade most assuredly should NOT have changed coding on records (i.e. it should not have changed data values on records).
One suspicion: An index may either have been messed up before the upgrade OR is messed up NOW, after the upgrade. Not sure if any re-indexing has been done before, during or after the upgrade?!?!? But...if she is seeing MORE now, I would suspect a previously bad index has now been fixed. She may have simply not been seeing them before due to a messed up index (just speculating).
Also, best practice protocol for upgrades is to do pre-upgrade SQL DB backups. Therefore, in such a situation where questions are being raised about the data AFTER the upgrade, you can always restore the pre-upgrade backups to a temporary SQL DB name and COMPARE the data in question.
Ideally, you should be able to PRECISELY nail whatever is the "question" at hand, by comparing pre- and post-upgrade data.
Hope this feedback helps.