Database merge question

Hi
I’m trying to merge two databases:
a) WP1.3-alpha-2, updated to 1.5.2 DB
b) old Pivot entries, imported into 1.5.2 DB

luckily, my post ID’s do not overlap, the same goes to categories, and since pivot didn’t have any linkmanager or postmeta, they don’t need to be merged…

but i have some questions about other ID’s:

wp_post2cat.rel_id
wp_comments.comment_ID

are they used in any manner exept for being an ID.
if i change them, will i break something?
if not, how can i reset them a la “give each row a new unique ID, start from number 1234” (let’s say, i have wp_comments.comment_ID’s 1-1233 in the first db)

TIA

sepp