You can replace sodium carbonate with potassium carbonate if you correct for the difference in molecular weight. Potassium salts are slightly more active then their sodium analogs in photograpic formulas. The one exception being for use in fixers where the potassium salts retard the fixing process.
Isn't the carbonate part only that is active? That is, pH is controlled only by cabonate, not the counter-cation.
I did know K+ salts are only a little soluble than Na+ salts. Maybe is this the difference?