As you may have heard, Blizzard is recycling many current glyph IDs which are being reused in Mists of Pandaria (as they did with old IDs from Wrath of the Lich King). What does this mean for you? Say you take my currently known Glyph of Spirit Tap. The corresponding ID in Wrath was Glyph of Mind Sear, and in MoP will become the new Glyph of Dark Binding.
Wait, what just happened? That's right, the old glyph I knew simply transformed into a new glyph at no charge.
A lot of you may be saying, "Well, duh!" If that's the case, this thread isn't for you. However if you want to know how to maximize this opportunity to get as many free glyphs as possible in 5.0, read on...
It's very simple. All you do is learn all possible glyphs for your character(s). Really, it's that easy. Pay what you normally would from the Auction House, scrounge up "bad" glyphs from your messy guild bank, and just learn, learn, learn.
Don't just take my word for it, however; WoW Insider mentioned this in their A First Look at Mist of Pandaria Professions: Inscription, seen here: http://wow.joystiq.com/2012/06/12/a-first-look-at-mists-of-pandaria-professions-inscription/
Are you wondering which glyphs are actually getting converted? If you're looking for ID to ID side-by-side comparison, check out The Stoppable Force's comprehensive guide by class: http://thestoppableforce.net/glyph-conversion-chart/
A few caveats: there may be some new created IDs, which means that you will not automatically know every glyph for your class. This will definitely be the case for monks.
If you're scrambling to find those few missing, obscure (and at the moment, likely useless) glyphs to fill out the roster, I can certainly be of service! I've spent ridiculous amounts of gold on Book of Glyph Mastery to learn every available glyph in the game currently. I also have a nice stockpile of mats and won't gouge the prices, which is certainly going to be the case when the expansion hits.
One last note... Your due date to complete this is likely NOT September 25th, but the upcoming Tuesday when patch 5.0 is released. Which Tuesday is hard to say, but with the background downloader running we all know it's coming soon.
You have limited time to save yourself a lot of money, so get on it, Azjol!