Here is what I believe is the correct way to do it, but I hope a guru steps in and speaks up so I don't mislead you...
First, create a new chunk and designate it for your logs, then create the logs (using onparams -a -d logdbs -s size if you want)...
Second, do a system archive to activate the new logs (ontape -s)...
Third, move the current log to one of the new logs (use onmode -l if you want, repeating until the current log is one of the new logs)...
Fourth, perform a checkpoint in the current log (use onmode -c if you want)...
Fifth, delete the original logs in the rootdbs (use onparams -d -l logid if you want, repeating until the original logs have been deleted)
I think that is it...
However, if I were you and don't want to take the chance of screwing up anything, I'd wait for more replies on your post from more experienced informix dba's...