Create a separate branch in your revision control system for your 6.1
project work. If you're not comfortable with installing multiple versions
of software on whatever-your-platform, install the newer version under
VMWare or equivalent so there's zero chance of profile conflicts, etc.
I just created 6.1 on the same machine, without any problem. Both the
versions exists without any problem.
Now... I will work with rails 2.0.2 with 6.1 and old Rails with 6.0
The New version 6.1, was installs and works without any problems.
Its faster than 6.0 and has many advantages.
I would recommend, to install 6.1 also,( Just 26 MB ) side by side to
work with rails 2.0 and 1.2 easily.
Hope this helps