The obvious answer is... it depends...
That would depend on what features your database uses and what you need. If it is pure data, with very little in the matter of triggers, constraints, simple indexes only, no stored procedures - then is it quite simply a matter of dumping and importing and can be quite painless.
If you have a lot of binary data, complex triggers or constraints, stored procedures... then it is a project and a half since you have to redo all those features.
Similarly, if your database is used by a front end which is not web based, or tightly integrated with SQL server, you might have a lot of rewriting to do on that front.