When I change pricing I always worry that current customers will be unhappy because they might have saved some money had they bought it later (I sell a desktop product).
Is this a problem you have seen youself when you sold desktop software?
I think you should give them the option to buy at the new price or stay on their current plan. Think about receiving an email from a service you have been paying $99 a month to that says, "Hey, we are lauching this new pricing at $79 - if it suits your needs you could save $20 a month"
Definitely is not the norm from the majority of SaaS sites.
We hate TelCos who lower the prices and even apply them retro-actively? That's news to me.
But more to the point: the logistics of making refunds on purchases like software are complicated. Most payment processors don't even have the notion of partial refunds.
Making a full refund is costly to you (payment processor took their cut of the transaction; they aren't too eager to give it back to you so either they charge you (and now you have a loss) or they swallow the charge but will kick you out of the system if they have to do it too frequently).
There might be more bad will generated by contacting the customers and offering them refund because that makes them aware of some event that they might not like.
Most people don't monitor the price of software they bought over time so they are not aware of changes so they are not upset about changes. I've bought a bunch of software over time and I can't tell how much any of it cost - I just don't keep track.
One day you find out that your parents are still paying $100/month for a 1mbps plan that doesn't even exist anymore, while they could pay $50 for 10mbps after a 2 min phone call. That's what we hate. Not everyone knows to check every couple months to verify that they're paying current prices.
This is on SaaS, so no refunds needed, just a price/plan change for the next billing cycle.
Is this a problem you have seen youself when you sold desktop software?