My experience tells me that it would not add or edit your current configs(most likely over write them) .It would be like having the wrong configs . Now this isn't a straight forward answer .If the mod is still in same mc version and no major changes have been made to mod in a update then it should work the old config file .Usually tho mods are constantly changing and so are the configs .So an updated mod needs updated configs .I can't help but , to think you are trying to do this because you edit the configs a lot and hate doing so every time.I know I do ,but that is modded mc if you want it to run as best as you can you need to proper up date and configs which means re editing the configs the way you like to play them . I hope this helps and answers your question . using old configs will cause problems the only question is how soon and how many problems will arise but, problems will show up eventually.