Just for completeness, it depend on what the distribution system is. Your comment assumes an mgn with an existing phase and neutral conductor. Most areas here are ungrounded Delta so you would already have two line conductors for a single phase service.
Dim lights may be preferred to VFD noise.
Constant VFD radio noise may be worse for LED lights, smoke or security alarms, energy efficient appliances, big screen TV’s, AFCI, and GFCI devices.
In my area, there usually is not two line conductors with a neutral, mostly either a single phase line, or complete three phase lines. They add a second line, and use only two transformers for substantial savings on small three phase service's.
That is the thing, single phase often is one wire at top of the pole and other one (usually the grounded conductor) a little lower. To go with three phase they need to add cross arms, whether adding one or two conductors and possibly use heavier pole than the existing one. The third conductor doesn't add all that much cost especially on only 1400' but all the other work needs done either way.
Around here, with most distribution being delta as i mentioned, the poles would already have crossarms, and I think to add the third phase they would just add an insulator to the middle. IF they were converting it to a wye then they would likely add the neutral down below the crossarms. But even without having to add crossarms and/ or replace poles, Im sure it still wouldnt be cheap to add the 3rd conductor. Adding a conductor to an energized and in use line has got to be a pain. I hear what they do is install a special certified non conductive rope and then pull the conductor in under tension.
If you are at the end of the line or have limited number of customers on said line they likely disconnect while doing this. If it happens to be able to be fed from two directions, they may feed from both ways and isolate the area they are working on. Two way feed probably isn't too likely though if it is a single phase line but does happen maybe more often on rural systems. Has happened to a few locations right around where I live simply because of improvements over the years. Only had single phase past my place when I first came here. Then three phase was added for some nearby irrigation services. More recently new substation was added to the area. Soon after an existing single phase line going one direction from the sub was converted to three phase and has ability to feed from either the new sub or if necessary can be switch to come from the direction it originally was fed from. Plus some other three phase extensions has left a single phase branch or two with three phase lines at both ends of the single phase branch. They likely wouldn't build it from scratch that way but since it was already there, you have some options when there is problems. Likely one end is not actively connected other than if deemed necessary
