In my case I used DROP_EXISTING to convert 2nd (primary key index) from nonclustered into clustered. So the whole sequence was:

1. Delete all foreign key references (ALTER TABLE Table1 DROP CONSTRAINT FK_Table1_Table2)

2. Drop 1st clustered index (DROP INDEX Table2.IX_Clustered_Column1_Column2)

3. Convert 2nd nonclustered index into clustered with Drop_Existing (CREATE UNIQUE CLUSTERED INDEX IX_PK_Column1_Column2 ON Table2 WITH DROP_EXISTING)

4. Create foreign key references again (ALTER TABLE Table1 ADD CONSTRAINT ...)

The problem was that foreign keys were referencing both indexes, I remember that I tried to drop 2nd PK index, but the server raised the same error with 'foreign key reference'.