It is surprising to see this coming from Spreedly, especially in the context of their previous discussion on the same topic in HN (when Chargify increased price).

When you price a subscription product, one should factor in long term sustainability at that price, for a long time.

Any price increase due to additional features / facilities you provide - say 24*7 on-call support, must not be burdened on your existing users.

Provide a way for existing users to "opt" for those at a higher price, but leave them at the current price as they stand.