Daniel,
HWS usually have a domed (convex) top and sometimes have a domed (concave) bottom. The sacrificial anode is usually inserted thru a (25mm?) hole in the top.

If you invert the HWS, the domed top, becomes a domed bottom.

If you use the HWS upside down you can use the Sacrificial Anode hole with a ball valve to allow complete draining of the HWS. If the element is half way up, this will not be a problem, but if it is at the bottom, you may need to heat the oil external to the HWS.

Some HWS have 2 elements, one at the top and one at the bottom, to allow the whole lot to heat off-peak, and just heat a little during peak periods. If yours has 2 elements, choose the element which is lower, when the HWS is inverted.
I hope this helps,
Tony