I think I can give a clear answer.
Tumbleweed works on a few Tumbleweed repos and a few openSUSE repos. For example I have these repos:
john@tumbleweed114:~> zypper lr
# | Alias | Name | Enabled | Refresh
--+----------------------------------+----------------------------------+---------+--------
1 | ChromiumTumbleweed | ChromiumTumbleweed | Yes | Yes
2 | PackmanTumbleweed | PackmanTumbleweed | Yes | Yes
3 | Tumbleweed | Tumbleweed | Yes | Yes
4 | Updates-for-openSUSE-11.4-11.4-0 | Updates for openSUSE 11.4 11.4-0 | Yes | Yes
5 | repo-non-oss | openSUSE-11.4-Non-Oss | Yes | Yes
6 | repo-oss | openSUSE-11.4-Oss | Yes | Yes
john@tumbleweed114:~>
Notice the first three are Tumbleweed repos and the last three are openSUSE 11.4 repos. When 12.1 is released I will simply switch the last three (the 11.4 repos) over to the corresponding repos for 12.1 – simple. Every few days I run the command “zypper dup”. When I change the 11.4 repos to the 12.1 repos I will simply run that key command once again.
(in the absence of any other advice that might be issued by Greg K-H).