This is not only the right thing to do, but you may notice higher retention rates in the grandfathered group. Now they're getting a great deal and may be reluctant to cancel.

I have a forum membership grandfathered at $8/mo from when I subscribed 7 years ago, which over the years has gone up to $50/mo for new members. Even if a few months go by where I don't visit the forum, I'm not going to let that thing lapse. :)