I just last month flipped an axle
on a tent trailer and here's some of the scoop:
IF the axles are dead on straight (no slight curve for some camber) you can turn the axle
over and bolt it onto the bottom of the springs, usually using the same shackles. However, if it is curved, you need to get a set of weld-on spring perches to put on the opposite side from the originals and than bolt the axle
to the springs on the top side of the axle. The latter is what I just had to do/...
An excellent trailer axle shop in SoCal is Arrow Trailer Supplies, 1363 Holt Ave, Ontario, 91762. They have an entire shop that does nothing but trailer suspension repair and modifications, and they are great people to deal with as well.