Is there a downside to upgrading to RoS asap

Solution 1:

Another minor, but more permanent benefit, is if you sit at level 60 without RoS you will gain Paragon experience. Paragon levels give all of your characters a boost like +Resist All for extra toughness, Crit Damage, Attack Speed, etc. And the first few dozen Paragon levels require less experience than it takes to go from 60-70.

You would normally gain Paragon levels upon reaching level 70 with RoS, so this isn't a strong reason to hold off of the expansion, but if you wanted to buff up your alts and have an easier time in Act 5, it would be a side-perk while you work on the older content.

The biggest benefit to this delay would be for hardcore heroes, where you want every edge you can get.

Solution 2:

Unless you actually looking to create those Hellfire rings, upgrading would stop and prevent you from completing those rings. There are also skill updates and gear changes would would normally break your current build. So if you want that Level 60 ring set, I would hold off on adding the expansion to your b.net account till you are finished with your rings.