No, in fact that's all you need to do. Halve the internal volume of the cab and halve the porting (which will maintain the current port tuning as stated by previous posters) and you've got yourself a 2 x 10.
If you're doing this for fun and to learn, it sounds like a good project. The good thing is that you could take all the parts like handles, corners, input panel, etc. from the 4x10, saving you a lot of hassle. You could certainly keep the tweeter if you wanted, but the crossover values will need to be changed. If you provide the crossover details, I'm sure someone on here will tell you what to do.
If it works out, you could then build yourself a 1 x10.
A build thread would be good.😁