When it comes to flightlevels, the first one "shall normally have priority", as stated in DOC 4444.
But look at DOC 4444 5.3.3.6 and 5.3.3.7.
But there are a lot of factors, and every situation is different. So the word here is "normally", it's still at the controllers discretion.
There are a lot of reasons why one would change the flight level of a preeceding, but it usually comes down to, what's easiest.