You say your father wanted the change so that his half of the house did not go to your mother. IF he told the solicitor this then, in my view, any half way competent solicitor should have asked him about the will - even if he didn't specifically mention it himself. (Apart from the fact that this is a matter of competence, the solicitor would surely have been interested in the will issue as a means of generating a little more income.)
However, as you will realise from my "IF", how are you going to be able to get the solicitor to accept that he/she was told the reason for the change? If your father gave no reason (or the solicitor says he did not & you can't produce evidence to the contrary) then you will have a much more difficult job getting anywhere. The only basis on which I think you might be able to have a go at it would be that an elderly person (I assume this applied to your father) would be extremely unlikely to go to a solicitor with a request to change the ownership arrangement without a good reason & the one most likely to apply would be to leave his share of the house to someone other than the joint owner. It could perhaps be arguable that this should have put the solicitor on notice to enquire about the reason. However, this is very tenuous.
I think all you can do at this stage is enquire of the solicitor exactly what happened - which should be recorded in their notes of the case. If you are not satisfied then you can pursue the complaints procedure (see Law society website).
Incidentally, there should be correspondence from the solicitor to your father - an engagement letter & statement of what the instructions were. What does that say? Also, was a power of attorney on behalf of your mother involved at all, or was the change made with only your father involved? Was it necessary for someone to sign forms on your mother's behalf? If that was done, who did it & what did they know?