Upgrade Karmic>Lucid on USB HD

Greetings from Vancouver!

I have not tried yet to update from Karmic to Lucid because I cannot find information about how to proceed in my situation: I have installed my Karmic OS fully on a USB hard disc drive, so I can keep a "dual boot" on my laptop (which has windows installed on its internal drive).

All works fine as of today: when my USB HDD is plugged in, grub comes up and I get to choose what OS to boot from.

I did this install on the HDD when it was brand new and from an ISO CD (windows was pre-installed on the laptop's internal drive).

My question is: can I safely upgrade from the HDD drive and not compromise either the USB HDD itself nor the internal HD with Windows on it? How should I proceed (upgrade from the running OS, download another ISO CD with Lucid, else...?)

Any help is much appreciated!