Recovery from 5.1 make installworld mistake

[cp]

I cvsup'd src=all, make buildworld, make installworld
and messed up my chance to build a RELENG_5_1
kernel. You all said to fall back to 5.0 and start over.

I went and got the machine from where it lives on the
net, brought it back to my kitchen (where I have only
modem access and began to do the fall back.
I'm at that point where I've passed the make installworld
with 5.0 sources and everything seems back in order.
Prior to downgrading all the sources to 5.0, I created 
a tar archive of /usr/src as it was after cvsup'ing 5.1.

Now as to getting back to 5.1. The Question needing 
opinions is: 

        Do I dare untar my 5.1 sources, overlaying
        /usr/src, and start my buildworld-buildkernel-
        installkernel-boot-installworld process or... 
        do I have to redo the cvsup? 

An alternate phrasing for the question is, will I be missing
anything meaningful by using my /usr/src/ archive of 5.1
sources? ports are virtually irrelevant on this system if that
is a factor and I've explicitly saved every meaningful 
tailored configuration (e.g. rc.firewall, rc.conf, hosts, etc). 
For me redoing the cvsup means taking the box
back to where I have no direct console access making
that first problematic 5.1 reboot difficult.

Thanks in advance,