can we change collation of the DB after it has filled with some data?

Yes you can. There are some things to check prior to doing it. This link has the information: ALTER DATABASE (Transact-SQL) See the section "Changing the Database Collation"

You can change the collation of a database at any time, but only new objects will have the new collation. To change existing objects you would need to script out each column of each object that has the collation (i.e. character fields) for example:

ALTER TABLE MyTable
ALTER COLUMN MyCol VARCHAR(10)
COLLATE NewCollation 

MSDN link