No, if your LJ account is deleted and purged and someone renames to the same username, they won't be able to manage your old imported DW comments! But it's still a good idea to claim your LJ OpenID: unless you claim it, you will lose the ability to manage, on Dreamwidth, comments that you left on LJ that were imported to DW and community entries you made on LJ that the community admin imported to DW.

In other words: claim your LJ OpenID now before your LJ account is deleted and purged so that you keep the same amount of control over imported-to-Dreamwidth comments left in other journals and community entries as you had when you originally made them on LJ.