GUEST wrote: ...tool (a flat pick will do this) to "unsnap" the top from the bottom...

The technical name for such a tool is a 'spudger'. Every time I crack open a case I wish I'd bought a set. Though yesterday I dismantled some headphones I bought in 1968 - before snap-together plastic was common. It was glued together. As for laptops, Thinkpads are easy to dismantle - mostly screws.

Easiest way to replace a hard disk, for non-technichal people, would be to download a Win 10 DVD and do a fresh install. Then restore data from a backup. Unless you no longer have program installation disks.

I wonder how the OP got on with Thunderbird? A common problem for people jumping from Outlook Express is the change from POP to IMAP. T'bird defaults to IMAP on a new installation if the ISP supports it, and the user doesn't understand the difference.