The time constant for the length of time activated. May be different than the parallel plugs require. Probably longer. Actually I know it is longer. So you have to mentally decide when you have had enough glow plug time. Or just wait until a much longer cycle than they require has occurred.

There is a differance in current draw I believe as well. The parallel plugs use a higher amperage fuse I think. My 77 300d will remain with the loop plugs as they do well enough in reasonable temperatures.

If the car was to be used in the winter I would change them. Actually when having loop plugs if one fails they all cease working. So a small kit to deal with this event should be in the trunk. While with the parallel system if one plug goes south the others remain in service.

Most important to remember is never test loop plugs with twelve volts. They are rated at something like 1.3 volts. They may or may not stand the abuse. So the only reason I see to upgrade the controller is to get a normal cycle time for the newer type.. Plus the controller may have a little heavier set of contacts. Although this is not known. Each type would have been designed either the same contact wise or not. Designed for more current than expected as an life expected issue.

The original controller does not sense what it is servicing. So the long cycle will persist if used. The only change it might experience is a greater current flow through the heavy duty contacts when using the parallel plugs.

Series plugs will also be far less tolerant of any additional resistance In their circuit. Many if those systems may have been upgraded because of age and cycling. Thermal cycling especially may have oxidized some of the connections in their circuits.