Hello first time on the forums. 

One of the departments here have a shared calendar with important dates logged into it that has to be referenced pretty often. The file is getting large and causing outlook to hang when searching through it.

What I want to do is make a new calendar using the same name then change the name of the old calendar so it can still be accessed.

what would be the best way to go about this? We are using Outlook 2010 clients with an online exchange service through Office 365.

Hi,

My suggestion in this scenario is to let the owner of this shared Calendar setup AutoArchive Setting on this calendar, thus old/expired items will be stored in a separate .pst file which won't be accessible by other users, the size of this calendar will also be reduced.

For detail steps, you can refer to this Microsoft article:

Delete or archive older calendar items

Regards,

Melon Chen
TechNet Community Su