YOU MUST provide a way to save message history for all the Groups
After getting everyone to believe in and use your tools for decades---the invaluable history of what people shared in messages is about to be lost. IF YOU DO THIS I WILL NEVER USE YOUR PRODUCTS AGAIN, AND ACTIVELY CAMPAIGN AGAINST YOU---IT IS UNFORGIVABLE....
Wilson Logan commented
There are tools available to preserve your data.
Might I suggest that you google "Personal Groupware" ?