Increasing number sequence

I am having a problem increasing the number sequence for Inventory Journals.

Largest is 999999, Next is1000000, Format is ######_61

I have therfore ran out of numbers

If I increase the Largest to 9999999, I also need to increase the Format to #######_61

Increasing Largest is OK but when I change Format I get the following error

One or more references using the current number sequence are not sufficiently long to contain numbers from this number sequence.

The string size for InventTransId is 20 so this should not be the problem.

Does anyone have any ideas?

Ken