I really don't recommend making major changes to your current drive. It would be nice to have a known good drive on hand when something goes wrong. Why not do the upgrade in the other sequence? WARNING: I don't have a RID box, so I haven't done the 4.x on RID process yet.
You'll need an LBA-48 2.4.4 (i.e. 3.1.1x software) kernel, and must do an in-line monte using the process outlined by NutKase, AlphaWolf, and others.
1. Use an LBA-48 compatible boot cd (PTV Upgrade LBA-48 3.01) and use MFS Tools to upgrade your 40 Gig drive to the 160 Gig. Before booting it in the TiVo, do the in-line monte. The first kernel will be the killhdinitrd 3.1.1c from the above cd. The second kernel (vmlinux.px) will be the 3.1.1 unscramble kernel (sorry, I don't remember who compiled it). This will preserve all your recordings, and give you the full capacity of the new drive.
2. Use AlphaWolf's slices to upgrade to 4.x, followed by the 4.x on RID update (must be done at the same time).
3. Add the rest of the hacks, etc. Voila! 4.0.1b on a 160 Gig drive in a RID system.
Alternatively, borrow a 40 or 80 Gig drive, copy your current drive to that, (do NOT expand), then follow the upgrade process you described. It is REALLY a bad idea to make changes to your original drive.
There's a difference between needing help, and just being plain ole' lazy.
"You cannot teach a man anything. You can only help him find it for himself." Galileo Galilei (1564-1642)
HR20-700 with 2 TB, HR22-100, HR22-100, HR22-100, HR23-100 all running 0x5cd and networked.