Upgrading from 1.2.3 to 1.9.x - Any issues?

[Jos]

I would try to do it in one step to 1.9.4.2

Make a backup of your database and files first.

Update your modules right after upgrading CMSms core.

[replytomk3]

should upgrade to slightly newer version e.g. 1.5.x and then upgrade again?

My recommendation is to do it in steps, checking how everything works and with backups on every step.