Don White runs http://www.cruisingelectronics.com , which uses the great OpenCart e-commerce open-source package, selling accessories to boat owners. Items include clocks, barometers, first aid kits, bells and sundry others.
Don was quite capable at configuring OpenCart, but was aware that his version was getting old. At that time, v2 of OpenCart was circulating but it seemed still too new to adopt for this site. This was probably over-cautious and if there had been "must have" features we would have explored the possibility further.
I wanted to be careful to have a smooth transiation and not to interrupt the running of the site for any length of time. So I cloned the site onto my laptop, and had a dry-run of everything first. The database upgrade script, which is supplied by OpenCart, worked perfectly. There were a few things which needed manual intervention -
System missing images, two boxes needed sizes
In table languages there was only english but status =0 (disabled) for reasons unknown
Then carefully follow instructions in beta script about "Admin->Extensions->Modules" - they really do put this help text there for a reason!
I also installed the popular VQMod so that the site could get an automatic detection of currency - it ships all around the world, not just NZ.
We did the real changeover just after New Year, when business is slow, and it took just a few hours. When e-commerce is involved, no matter how many payment tests you do with a testing sandbox, nothing beats the final proper test with Live transactions. I always create a a ZZZTest product which is 10cents or so, and buy that. If that works, it will work for any amount.
There were some later tweaks for CSS and handling tax. Tax always puzzles me in OpenCart... which I must admit is a poor reflection on me, not on OpenCart.