Ads get purged if they expire - AUTOMATICALLY. Thank god for that.
If you are not receiving email because your ads are about to expire it's probably because your mail host is rejecting it. That doesn't mean junk mail filtering. It means totally rejected. Think AOL, Comcast sometimes and some others.
No ads were purged manually and none would have been affected by the server change. I suspect your ad expired because you did not relist it when it came time to expire. Whatever the ad was.
BTW - "ad" as in advertisement.