if you are considering upgrading from relative dated releases such as 4.3.4 or 5.1.3 or 5.2.x to a new release and namely 6.3.2 onwards then there are some things that you need to know.
Because Cisco for security reasons decided to change the image signing and in 6.3.2 onwards this new signature is used one might have an issue upgrading directly from these "older" releases directly to these new signed versions.
The "official" procedure is to move from say 4.3.4 to a "dual-signed" image such as 5.3.x or 61/62 and from there migrate to the desired release say 6.4, but this is a 2 reload sequence that I can't suggest a normal person!
For this reason we have the option to install a bridge smu that accepts any signature, so one can go directly from say 4.3.4 to 6.4.1. This signature override bridge smu is only needed for the upgrade scenario and after the upgrade it can be tossed and is no longer even used, so there is no security concern as such.
we produced a few of these engineering bridge smu's already for designated versions, but if your from release is not available, then we can surely provide one not a problem to ease the migration to a more smoother and less time consuming procedure.