You can install iOS 15 now. However, when you get your new iPhone you might first set it up as a new device (i.e. no backup) then update iOS since the 15 installed may not be the same as today's release. Then once updated, install your current iPhone's backup. This has been my practice for the last several releases, I feel it's good practice to flash a new copy of iOS on a brand new device, since they are sometimes behind and would need to be done anyway.