I see a lot of useful replies here.

I prefer to answer to 4 simple questions, helps me have a clear idea without overcomplicating:

• What success looks like. Be concrete: “in a year I want X”. If you can’t say it, probably it's not the right time for the decision.
• The risks. What could go wrong, and how bad would it be? Money, time, people affected.
• The known issues. The problems you can already see today and are choosing to accept.
• The way back. If it goes wrong, how do I undo it, what would it cost, and when will I check? Put a date on it, like “in 3 months I review this”.

The fourth point helps you to determine how "big" the decision is. If it's easy to reverse, decide fast. If it isn’t, slow down, sleep on it, and talk to someone who has already done it.