My ISP is changing upstreams for a variety of reasons around May 18th.
That is unfortunate.
1. I have a /24 SWIPed to me, 205.238.207.0. The ARIN listing doesn't say it's non-portable, so can I take it with me? And if I do, how likely is it that the new ISP's announcement will get installed in backbone routers rather than filtered for being too long? It's right in the middle of the old upstream's block 205.238.192/18.
If you *own* the block, you can do anything you want with it. I don't think there is an issue with with routing, you just have to do it. Then again, I may be wrong.
2. Assuming I have to renumber, I have a whole bunch of virtual domains all DNS served from one server in the renumbered space called ivan.iecc.com. (There's other servers elsewhere that won't get renumbered.) If I tell the Internic that ivan's host number has changed, will that update all the domains served from it?
Sure, a host modify (http://rs.internic.net/cgi-bin/itts/host) will set you up. If you can, I'd try to overlap service so when things fubar, you can always fallback to pieces-parts 'till you can figure things out :) -ravi