Yeah, delay after switching feeds is normal, but honestly most people fixate on the wrong thing. everyone panics about Google Ads reprocessing, but the real kicker is almost always a dumb formatting mismatch in custom labels. i've seen it a hundred times - the new app spits out "Label_1" instead of "Label 1" or something equally asinine, and suddenly your Shopping campaigns look dead because the product group filter no longer matches. Google won't flag it as an error either, just silently stops serving.
also, double-check the feed destination and country mapping before you waste hours waiting for a sync. Swapping apps can mess with those subtle settings, and campaigns won't pick anything up if they're targeting a different Merchant Center account or country code. I usually tell people to audit the raw feed file in a text editor before even looking at GMC status. Most cases recover within a few hours if nothing changed, but if your label values actually shifted, you'll need to manually update campaign filters - no way around it.
