First go to the APP store and see if both Lion and Mt Lion are in your account. If that is the case, then you can always install either when eve you wish. There several options that you can exercise to guarantee this.
1. Partition your internal HDD and install Mt. Lion on it (a small partition will do). In the event that you would want to use it, you would have to clone that partition on to a larger HDD then transfer your user data to the new HDD. Then you would have to install it into your MBP by a physical swap or clone it to your internal HDD.
2. Get an external HDD and install Mt. Lion on it. Same directions as above, but omitting the partition of the current HDD.
3. Make a USB thumb drive of Mt. Lion. This would allow you to install Mt. Lion over Lion on your internal HDD without any cloning or user data transfer. I have made USB thumb drives for both Lion and Mt. lion. This way you do not need an Internet connection to install the OS and always have an option to revert to either OS. I consider the best alternative.
Here are directions for creating the USB thumb drive (They also apply to Lion):
http://www.macworld.com/article/1167857/how_to_make_a_bootable_mountain_lion_ins tall_drive.html
I like Time Machine and it is an excellent backup medium. With that said, I like redundant backups. In addition to Time Machine, another external HDD with all or the critical data is good to have for the simple reason that all HDDs will fail eventually. Some people like to clone the internal HDD which creates a bootable HDD if the internal HDD should fail. I don't do that because the USB thumb drives essentially perform the boot function, so I opt to just backup the important files.
I hope I have covered everything. If clarification is needed, don't be bashful. Just ask.
Ciao