you need to get rid of old prices in area completely first. GS with prices in gallons are preventing other GS to allow correct prices in liters.
we were having similar problem in Prague when gas prices were started in czech republic.
there were several dozens of GS in Prague which had N/A prices and we couldn't enter correct prices with same error.
there were also several (maybe 4-5) GS with totally incorrect prices (3,5 vs 35,5) for which we just couldn't enter correct prices.
but then one guy just deleted all the wrong prices and from that (or next) day everything worked.
so delete prices for all GS which got prices in gallons. to be able to do it quick, you may use mock locations on Android client (app Fake GPS