Upgrade dilemma

["Westdorp, Tom" <Tom.Westdorp@xxxxxxxxxxxxxxxxxx>]

 We are doing a little upgrade project, and I'm going to lay out the basics
here and will be happy as can be for any suggestions to improve the process
with regard to speed.

We've got two 4-way 820's each on v5r1 and each with an SPD tower with disks
that are used.
We're going to two 825's which of course means v5r2 and no SPD tower.
Currently the boxes are at separate sites for DR.

Goal is to perform the upgrades with no downtime, at no cost and with no
risk.  <guffaw>

Current plan is to upgrade the current backup system to v5r2 and 825.
Co-locate the systems at the production site.
Move as much 'static' storage as possible using Tape (3590 E-11) while users
are up.
Call for some midnight down time to do checkpoint for Save While Active and
checkpoint for Vision OMS, then let the users back on while we finish the
save and do the restore.  
Current plan is to use the 3590's (we have two) to transport the data.
        We are told we can't hook the two systems together via HSL because
of the SPD tower still in the config of the running system.
        We are told we can't hook the two systems together via crossover
cabled gigE because of the v5r1 status of the running box.
        That leaves us with just 100m ethernet, or the tapes.
Once all the restores are complete on the upgraded system we apply all
transactions through OMS until both boxes are in sync, and then in the
middle of the next night we get downtime again.
Then we switch the users to the upgraded system, and start queuing
transactions via OMS to send back.
Next we upgrade the second system to v5r2 and 825.
Then we turn loose the OMS transactions and get both machines synced.
Some future night we get downtime again, switch the users back to the
regular system, and then let the users back in and queue transactions using
OMS.
Relocate the DR box to the hot site.
Resync with OMS.
Voila!  That was easy!

Does anybody see any major flaws?
Is the information we've received correct regarding the use of HSL and gigE?
Would HSL or GigE give us better throughput on the data transfer anyway?

We're a bunch of casinos, so we need 24x7 or as close as possible.  We
'only' have 1TByte of data to move...

Any pitfalls you folks have encountered in similar folly that we should be
especially alert for?

 TIA for any help anybody can offer!

Tom Westdorp