The way I see it: In the ur-priest entry it clearly states 'If such spellcasting ability was possesed, that ability is forever forsaken'.
So, you take first ur-priest level, you get divine spellcasting, you stop qualifying for ur-priest, you lose spellcasting. Regardless, of the fact that you now qualify, you possessed the ur-priest casting before, and therefore it's 'forever forsaken'.