Guild breakups involving significant number of people always result from differences in opinion on how a guild should be managed, not single-case minor problems like loot etc. Even if the difference was "slight", it warrented people leaving the guild and you can be sure that time spent seperate will only inflate things. If/when they come back, you should be really prepared for a major disruption. Which seems to be the current case.
Personally if i was an officer, i'd make real sure that the old timers werent going to be trouble before ever inviting them back. Since that hasnt happened, i'm guessing your current management is actually happy with the change.
TLDR: Oil & water dont mix. If the new old-timers arent leaving (dont expect them to change), then it's time for you to leave. But since it seems you have a small following of your own, do some preplanning & leave as a group, setup a new guild to continue your old habitat.
(We had a similar situation. Our founder had stopped playing the game but wanted to come back later. He's extremely respected by just about everyone and a very commanding presense & we were all jubilant to see him back. However, it turned out that we had moved in seperate directions & now our ideas on how the guild should be run didnt match. Fortunately for us, he was just one person and he was intelligent enough to see how things were and he left gracefully without anyone getting upset, including him)