Well, nobody goes from United to Liverpool or City, unless you’re 12, but it’s perfectly fine to fall out of love with United and generally just love football - if at all. Become a more objective viewer. Many foreigners are called glory hunters and I believe United of course have the most of them in the world given the long success over 20 years.
Will however a top red living in Manchester who goes to every game get more excited over a win or a trophy vs. some random Korean or Norwegian who became a fan after Solskjær scored against Bayern? Will the emitions fly higher? Of course not, it’s just football, which compared to a lot of stuff is pretty low on the list. The older you get and the more family members you have, as well as responsibility at work, the less time and effort is devoted to football. That’s my experience at least, I’d much rather watch PSG - City than United - Everton on a Wednesday night with this current bunch, gives me very little.