I romanced Liara in Mass Effect 1. In Mass Effect Two, however, I romanced Miranda as well. In Mass Effect Three when Liara asks if were still together in the beginning of the game and I said we should just be friends, will I have another chance to be in a relationship her later in the game?
I declined her in the start of the game. And we ended up as a couple of sorts in the end.
I would assume that if you follow it up and pursue her romantically, you should be able to roamnce her just like a character who didn't romance her in ME1 can. The options to lock in the ormance are pretty far in, so I woudln't think this early response would lock you out.