I made it clear to the sales rep that I wanted a seamless transition for my internet, or at least for him to guide me through the process so I could minimise the disruption.
Xnet's sales have given you the correct advice.
Orcon (on Orcon equipment in the exchange) to Xnet (on Chorus equipment in the exchange) on the same line is not going to be seemlesss with no downtime, nothing Xnet can do to change that.
It involves tech's for Orcon/Kordia removing the line from their gear and Chorus techs to connect/activate the line on their equipment. This can't always happen on the same day at the same time because it involves different techs from/on behalf of two different companies.
Most houses/premises have 2 pairs, ie: 2 lines into the place. Keeping the Orcon connection active on the 1st line while setting up Xnet on the 2nd line is the only way to give you no downtime. Because the 2nd line probably isn't used and may not be wired up to anything you need the full connection and wiring fee to hook this up to a jack point (unless you know it's already hooked up?).
If you didn't mean no downtime or can use mobile broadband as a stop gap for a few hours (best case) to a couple of days (worst case) then just go with the changeover on the 1st line from Orcon to Xnet.
I'd actually applaud Xnet's team for being honest and knowledgeable, other ISP's may have not understood the full effect of a change from LLU to Chorus equipment and told you "she'll be right" and hoped for the best in the transition.. you'd be far more angry if you switched to a different ISP providing over Chorus equipment and had a couple of days downtime after they told you there would be no downtime.