Should a single hand fiberglass rod be converted to a switch rod if the original rated line weight will be used? My question is whether it would be "safe" for the rod with two hand use. I realize it would not perform as well as a switch rod built and lined for two hand use.

To me, I don't see a problem ..... with the proper line and no double hand overhead casts for distance. It seems to me that the rod is moved at about the same speed with either one or two hands. The line would be lighter and thinner than a conventional switch rod line too. Having less mass to accelerate and less stick to the water the load on the rod might even be less. Also, the rod will not be exposed to the added stress of hauling when being used with two hands. I know most two hand rods are built heavier at the base, but they are also using heavier and thicker (more water load) line.

Many years ago, when switch rods were first coming out, I purchased a 10'-8", 7 wt from Batson. They specified that I was to use 7 wt, weight forward line. Switch rod lines were not available. No mention was made of using 2 or 3 line weights heavier that the rod rating as some manufacturer do today. I suspect this blank was just a conventional 7 wt. design, just longer. I still have it and no breaks. Does anyone know if it was "beefed up" to take any additional load due to casting with two hands?

I would strip seat and grip off and install a REC RSLS seat with matching removable fighting butt hardware which will extend the rod length by 2".
Build a 5" lower grip and use male part of the REC butt hardware to attach. This way you will not have to worry about extending the grip towards the tip which would have put an excess load above the current grip which is your real risk.
Herb

I assume you plan on extending the rod to accommodate the adding of the lower grip. Depending upon the rods action you may or may not be happy with it's performance if you intend to do Spey type casts, but it should be "safe".

Phil,
I agree.
I can not see any advantage to do this unless there is a physical issue
Herb