I tried canning spiced peach jam last night with a friend, and the recipe she found called for the inversion canning method to seal the jars. I should have researched on this last night, before we canned some of the peaches... but I didn't. I haven't gone to check if all of the tops sealed properly yet, but I am wondering if it's possible to water bath them after they have set. Will that ruin the jam?
Yes you can reprocess them in a water bath within 24 hours, and it should not ruin the jam. According to National Center for Home Food Preservation .
Reprocessing Unsealed Jars: If a lid fails to seal on a jar, remove the lid and check the jar-sealing surface for tiny nicks. If necessary, change the jar, add a new, properly prepared lid, and reprocess within 24 hours using the same processing time. Headspace in unsealed jars may be adjusted to 1½ inches and jars could be frozen instead of reprocessed. Foods in single unsealed jars could be stored in the refrigerator and consumed within several days.
If you want to reprocess all the jars, even if they seemed to seal, make sure you remove the lids and continue from that point. Instructions for water bath canning can be found here.