You're not alone on this one. I started without a contract to. I think a good number of us have done it.

If I were in your position I would outline my expectations in a contract/policy's and let the clients know that due to your current business plan you need to address some things that you haven't been so firm on in the past.

You also need to be prepared to loose some of them. The reason these clients went with you is probably because you were one of the few that didn't have a contract and were/or seemed flexible to them.

I'd give them 30 days notice of the changes.