You are quite wise to do so..
I can't tell you how many times I've seen people end up with green hair due to tinting back... it does happen in certain situations!!!
If you are going from a medium ash blonde or lighter to an ash brown, YOU WILL END UP WITH GREENISH HAIR. The reason is that there is not enough red in the hair to balance the high level of ash pigment.
When tinting back from blonde to brown, NEVER use an ashy brown.