Hi Mark, as you want. I agree that it should be fixed both for 1.7 and 1.8. Steps should be:
- Change the definition under 17_STABLE and HEAD. This will fix installations.
- Upgrade 17_STABLE to convert the field to NULL and to drop the DEFAULT (two steps).
- Upgrade HEAD to convert the field to NULL and to drop the DEFAULT (two steps).
As lasson->version are the same both to 17_STABLE and HEAD, incrementing both to 2006091802 should be Ok and there isn't need to perform any conditional execution under HEAD.
(Note that the "drop default" can fail under some servers so I wouldn't add its return value to the $result variable).
You decide! 
I put 1.8 as the fix version, but this should really be 1.7 STABLE because the Lesson module does not work at all on a fresh install using the install.xml file.