Hi Stephen,
I don't know of a way to do it automatically, but you can do it manually.
If you create a smart list to show all those leads who had the value changed (Data Value Changed Filter), you can then click into the leads and see what the value was before the the change was made in the Activity Log. You then either update the field from the lead record, or run a smart campaign to update the data to what it was before the error. Unfortunately, this only works well if you have a handful of leads (as you have to do it one by one), but if you've got hundreds, it might be better to do it within your CRM system as a bulk update.
How many do you need to change and what CRM do you have synced to Marketo? I guess you could also do it in bulk as a list upload into Marketo, but you would still need to see what the value was before the change.
Natali