Solver Variables, only change if it still needs solving?

it will depend on the details, and there could be multiple ways to do this. I am not aware of some kind of simple "toggle" in Solver that will do this, so it will take some more detailed work.

I could not tell if your question was "if a given 'by changing' cell's contribution to the solution is already known, don't include this cell in the by changing cell". If this is what you mean, I would probably be inclined to handle this in the spreadsheet. Have two ranges for the decision variables. In one range, calculate the desired variable(s) with an adjacent toggle value(s). Another range for the Solver "By changing cells". Have a third range for the main inputs to the Solver model that is populated by a simple IF() function. IF(toggle is true,value from calculated decision variable range,value from solver by changing cell range).

We will probably need more detail and maybe even a good example spreadsheet in order to really provide some good suggestions.