It depends whether you think of these as version numbers or not. If they are *numbers* then they need to go up, and so you can't go from 1.9 to 1.10, as 1.10 == 1.1 which is less than 1.9. Instead, go from 1.9 to 1.91. Then from 1.99 to 1.991, and so on. Or You could go from 1.9 to 2, then 3, then 4, and so on.

More importantly, I suppose it depends on whether your users, auditors, or software think of them as number.