I wanted to merge two keywords (eg tag1 and ta1g) but renaming the second with the name of the first, the first tag was removed!?
At the same time new feature suggestion alphanumeric sort keywords
Unfortunately, it's not possible to merge two labels. The only workaround to do it would be to view all tasks with a certain label and then edit each, add the other label to every task and delete the other label.
We'll look into implementing such options in the future, though and you're welcome to request them in our Votebox: http://todoist.com/Vote/