Thank you Schis for your reply!
I am afraid after further investigation I came to the same conclusion.
I am a bit puzzled now about how to proceed with that.
Currently I host a 1.6.4 MCPC+ server with over 100 mods. I did not start with 100 but the system (and the worlds contained) evovled over time. I developed a set of scripts that would all my clients allow to automagically update all their mods and configs according to the servers requirements whenever they logged in. With that I was able to play around with configs and IDs as needed but I could be sure that the clients would receive exactly the configs (and IDs) I had prepared for them.
All that seems to be not possible anymore. Should I start a new server based on 1.7.2 I can not be sure that introducing or even updating mods would be harmless in that regard since I don't have any influence over IDs (and the potential conflicts) anymore.
Not sure what to do now ...