I decided I was ready when my second plane both became boring, and had traits that I felt were limiting my flying capabilities and/or progression. It's really up to you to decide when you're ready. The first thing that comes to mind to watch for is stall characteristics. The plane may stall at higher airspeed and may be very unpredictable in a stall. As always it's a good idea to get the plane up high and practice stalls. There is absolutely no shame in asking to be on a buddy chord while learning your new plane. You may feel totally comfortable with it and never use the chord again, or it may feel too twitchy and unstable, in which case the both of you can land the plane and decide if you want to keep it on the back stove.

For practice you can get your current plane up a few mistakes high and practice stalls in all different attitudes, even inverted, and practice recoveries. I had the experience of getting a plane in a stall and not knowing how to recover that particular plane. It crashed but was not lost (luckily...a bit of devine intervention on that one!) but this was something I wish I had practiced more.