Replacement tire question....

1.  Trailer tires should be replaced every 15,000ish miles or 4-5 years, whichever comes first.  The reason is that on a trailer, the tires are an "active" part of the suspension.  That is to say, they flex much more than a passenger vehicle tire, thus the carcass of the tire usually wears out before the tread shows any wear.  The year recommendation is simply due to weathering of the tire (age cracks, etc...) that may occur over time.  If you have less than 15,000 miles and see no evidence of weathering, it is safe to use the tires longer than 5 years.  A lot depends on the storage of your trailer.

2.  It is not beneficial to purchase a higher ply tire than originally came on the trailer and in fact can be the opposite.  Again, this is because the tires are an "active" part of the suspension.  If you increase plys, you increase the sidewall stiffness and reduce the flex which results in a rougher ride for your trailer and may result in poorer towing behavior from your trailer.  If, however, the tires are not stiff enough for your trailer, the sidewalls will heat excessively and may blow out.  As far as rating, stick with whatever came on the trailer from the factory.

Having said that, I regularly go over the 15,000 mile limit, but then again I tow more than 40,000 miles in a year's time and a vast majority of my miles are highway.  Generally speaking, I replace my tires once at the halfway point of my year.  I also regularly tow at speeds up to 75mph.  I know speeds of over 65mph are generally not recommended for trailer tires, but I have yet to have an issue.  When purchasing tires, I go with whatever brand the tire shop has a deal on at the time.  Again, (knock on wood), I've never had an issue.

As the tire size difference is negligible, I would rather get actual trailer tires (as opposed to car tires), as it would seem the rig would tow better and be safer.  Most trailer tires are 10-ply (as opposed to 4 on the current ones), and have a better payload capacity.  Think I'm going to go with these Hercules that I can get locally:  https://www.amazon.com/Hercules-Power-75R15-117L-68146/dp/B006UH6U9M