First off, there is no LMDE-9 or 10 . . . there is simply LMDE.
LMDE is based on Debian and not on Ubuntu.
With Ubuntu the recommended way to upgrade to a newer version is to do a 'fresh install'.
With Debian the recommended way to upgrade to a newer version is to 'update'.
With the 'rolling release' of Debian, once you have it configured the way you want it ... you should 'never' have to do a fresh re-install.
So, NO ... an 'update/upgrade' within a Debian (or Debian-based) OS, should not break your system.
LMDE is based on Debian Squeeze/testing, and at the moment Squeeze is so close to being released as 'stable' that it is already solid stable.
If, however, you are using Mint-9 or Mint-10 (Ubuntu-based) ... proceed with caution.
As Ubuntu upgrades have been known to 'crash' from time to time
I have never had a Linux OS crash or break on me, that was not self-inflicted.
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