Upgrades

Hi,
Surprises are really nice … when it’s your birthday, your anniversary, and/or Christmas.
However …
I’ve just been through 9 long months of #$%@ getting my site up and running. I haven’t even had my GRAND OPENING yet! I’m still doing content!!!
But, here I sit, looking at upgrades for both WP and my theme.
Please, to avoid nasty surprises, and a lot of rework, just exactly what will I lose if I do these upgrades? And, what can I do to avoid the inevitable nasty surprises?
Thanks, DLC