To be honest, I'm not sure either. But from what I understand, when Mato quitted PokeSpecial due to health reasons, she wasn't just taking a break but actually resigning.
The PokeSpecial comic was sort of forced to stop if I remember correctly (at least for a few months). Everyone thought that would be it for the unfinished story until Satoshi (the current artist) agreed to take up where Mato left off and began with the Crystal plot.
So, in a way I suppose Satoshi is there to replace Mato but not to substitute for her, and I don't think Satoshi was recruited by Mato to fill in for her either but actually applied for the job independently. Thus, even if Mato wishes to come back, to certain extent Satoshi owns the right to drawing PokeSpecial now since Mato gave it up herself in the past.
Coronis~