Autodesk is currently offering 30% discount on Fusion 1 year licences. This does not seem to be conditional on having an existing seat. Discounted price is GBP462 (GBP385 + VAT).
There is no equivalent for 3 year licences which is a shame. They remain at full list (GBP1645 + VAT).
I tried entering the promotional code from the 1 year offer on the 3 year but this was rejected (suprise suprise).
The 3 year allows you to align the new purchase with an existing subscription renewal date but you can’t do this with the 1 year.