If you are talking purely about cash amounts the difference is £440 of extra income can will tax free for the tax year in question. 647L is worth about £88 extra in your pocket for the year. 603L= £6,030 & 647L = £6,470 (give or take a fiver each way).
The number part is your tax free allowance - ie £6470http://www.hmrc.gov.uk/incometax/codes-basics.htmI'm guessing 603L was last years allowance. So you should pay slightly less tax on 647L
647l