[xethos]

One at a time.

If you have the time and are interacting with the account, take the second to change it away from gMail. Insisting on doing them all immediately is setting up a Herculean task that'll almost certainly leave you demoralized. This gets your most-used (and presumably most important) accounts first,and feels much smaller and more manageable.

After 6 (or 18, whenever) months of this you can summon some motivation to change over the last 20 accounts and be done with it.