I would be worried as it it most likely that the boxing panel was corroded and the plate was welded on to "repair" it.
My advice is be prepared for the worst when you remove it
Richard
Opinions are like people,everyone can be different.
Certainly doesn't bode well in terms of the level of bodgery your car has suffered, but many have been brought back from similar or worse. You'll probably find fairly serious corrosion to the sill step as well. Recommend that you spend some quality time with the resto threads posted by Taupe or Neil MG (there are others as well!). And don't be tempted to do too much cutting too soon - even well rusted bits provide valuable reference points.
Looks like you need a new boxing panel and also an under outer sill. If these have gone you may also find the front and rear floor edge panels need work too!