Hello David,
we use IS-IS and I think this was done sometimes.
If you have a L2/L1 IS-IS adjacency this means the nets are already in the same area.
I would suggest to verify the following:
take the isis database
sh isis database
look for the presence of an IP default route 0.0.0.0/0.
The router that will become L1 only will lost all L2 information so it is important to verify that it will be able to use a default route to reach all the L2 prefixes.
In theory there is a bit that is set on the L2/L1 router the attachment bit but someone reported it is used only for CLNS routing not for ip routing.
So after all these checks are done you can decide if it is safe to proceed to the conversion or you need some node to inject a default route.
you can declare the L1-level at the process level (router isis) on the router that will become L1 only.
You can emulate the effects of this by setting the isis circuity type to l1 on the L2/L1 router's interface.
The adj should be rebuilt as L1 only.
You can check with
sh clns int type x/y
After the change verify you can reach destinations that are not present in the L1 only router isis database.
Hope to help
Giuseppe
