Hi @Jonty Morris
The first thing we should clarify is there is a difference between the deployment model classic vs arm and the storage version v1 vs v2
You can upgrade your deployment model from classic to arm and this wont impact the storage account version.
There are changes in billing from v1 to v2 and this article https://learn.microsoft.com/en-us/azure/storage/common/storage-account-upgrade?tabs=azure-portal#pricing-and-billing has quite a lot of detail on how to gather usage metrics and estimate costs.
One benefit of moving to v2 is you will be able to make use of the lifecycle management features described https://learn.microsoft.com/en-us/azure/storage/blobs/lifecycle-management-policy-configure?tabs=azure-portal
Sounds like it could be useful in your scenario