changing the collation of the database is only a tiny 1% piece of the puzzle.

every varchar/char/nvarchar/nchar/text column in the database has it's own collation, that is not affected just because you change the database collation.

you have to change the individual columns as well, which can get very complex if defaults, check constraints, indexes or several other things are applied to any of those data type columns.

the default collation only affects newly created items that do not explicitly define their collation.

i made a script to change collation once, it worked for my situation, as i remember it it was a monster at 600 lines of code that took all those things into consideration.