AccountHistory Object level, the OldValue/NewValue fields are of type String, so you will not be able to perform any type of aggregation; despite only displaying what appears to be a single data type (number).
To acheive what you seek, you'll need to build a custom field on Account and name it Prior Value. From here, you can create a simple Workflow rule which will update the Prior Value field with the old value on change.
If you are wanting to track Revenue for the Account, for n number of months (really any time period), then I would recommend creating a Custom Object named Revenue Transaction. So, instead of having users change a value for a specific field on the Account, they would simply enter a new Revenue Transaction under the Account. This way, you will know exactly Who and When an entry was made (great for auditing), and you can more effectively manage the data.
You'll need, at a minimum, these 3 fields:
- Transaction Date
As a bonus you can make the relationship Master-Detail when creating the Custom Object, which will allow you to summarize the Total Revenue at the Account level, and the entire Revenue Transaction table is fully reportable out of the box.