Hi I have switched from British gas/ dual fuel, to NPower this was a 18months ago, I had the same problem with them, although paying by direct debit, i had credit on both gas & electric, when i switched low and behold the credit turned into debit. at first I argued with them, boy have you ever spoken to British gas customer services? they really are the pits! in the end as i was getting nowhere, and don't know where to get advice,but if you do, then please let me know. i paid the so called arrears, just to get away.
Now i am about to switch from npower, who, incidentally, don't read meters, so I want to know who's been sticking their heads under my stairs in the last 18 months, however on my gas account i have £46 debit, on my electric i have £229 credit and my monthly D/D for that has gone up to £59 per month, crazy eh? if i have credit of this amount surly i should have a little payment holiday? No.
I bet when all the paper work goes through there will be debit, just as you are experiencing.
What i am going to do is get the meters read, on the same day change the suppliers that way you find out if you have any debits.
Now i have got all that with the answer to your question is: No, in a word.