Do a google search on "definition of drop frame timecode".
Your source timecode has absolutely NOTHING to do with your sequence time code. It's irrelevant. Your sequence is a separate entity removed from it's source components. The kind of code you used to create a sequence doesn't matter. Both drop and non drop can happily live together in a sequence.
I don't understand what you mean by changing the the timecode. If you're doing that, then you're breaking all connection with the original footage, making it impossible to ever accurately redigitize that material. I'd suggest you get someone to explain to you the use of timecode in the editing workflow, 'cause you're headed for a disaster!
Good luck.
Mark