Ahah! Time for Druid Rules-As-Written syllogism #2:
Major Premise: Only druid PCs have the druid PC class features listed in the druid PC class description.
Minor Premise: This PC isn't a druid.
Conclusion: This PC doesn't have the druid PC class features listed in the druid PC class description.
After all, there's no provision in the RAW for "ex-druid" PCs retaining any of the class features of druids, so what else could the RAW answer be for a PC who isn't a druid than they're treated exactly like any other PC who isn't a druid? Characters who aren't druids don't have druid class features.
It is absolutely true that the game doesn't have any rules specifying consequences for druids wearing metal armor. That's rather my point; it means there's nothing offering an alternative to the lose-all-druid-class-features logic. RAW, "druids will not wear armor or use shields made of metal". If your PC will, you've demonstrated your PC is not a druid. If a PC isn't a druid, then that PC doesn't get to have any druid class features.
Now we hit a point that is neither defined in the rules nor an obvious logical deduction from them, because you have a PC with an XP total higher than is compatible with their level total. Now you reach a point that can only be resolved by a house rule. One possible conclusion would be that you lose the levels and XP that you applied to druid (and you're at 0 levels and 0 hp if you didn't have levels in any other class). Another would be that you insta-rebuild with another class (or more than one) at your same level total. But, as best I can tell, there's no clear RAW syllogism to be applied at this point.