I wrote recently about capacity planning, with an item near the end about disk usage. Someone pointed out to me that compression can dramatically affect your baseline, and as you implement it, or de-implement it, you need to update your baseline.
That’s an important point, and if you make changes, you want to do one of two things.
Whether you restate the history is up to you, but I wouldn’t. I hate changing the old values, but I can see someone not being aware of changes and perhaps getting confused. If that is a problem, and you are sure you’ll keep compression around, then you can run a quick test of compressed and uncompressed backups, get a rough idea of the ratio, and then alter your old numbers.
For me, I prefer to make a documentation note, and update everyone to be aware of the change, perhaps even adding filters to my extrapolation routines to not use the older data.