A switch to RB300 arm on alexandria?

Well, I'm not an expert mind you, but I believe that you may not need a new arm board. What you need to find out is the pivot to spindle distance of the two arms. If they are the same, you're in hog heaven. What might be a problem for you is the difference in the weights of the two arms - if they are substantially different you may need to change the spring(s) or at least adjust them. Lastly, as I recall, the Oracle arm has VTA on the fly (or at least easily adjustible VTA. I don't recall the Rega arm having this option. Would you have to buy an add on adjuster? If its not important to you - its a moot point. FWIW, much as from an operational standpoint the Oracle arm is a PITA it is otherwise a good arm. Do you really think its worth the additional edxpense?

I recently did this. My previous arm was a Sonus Formula IV. I remade the arm-board (oak was the best match aesthetically). You can download a Rega template (if you do not already have it) from "The Vinyl Engine. (I just checked and it is temporarily off-line due to hacking)
I my case, the pivot centre was quite different so I needed the new arm board. You may not. If you check the specs it should tell you whether the hole is the right distance from the spindle. Also whether the hole is the right size.(23mm) I never had the original arm but I assume that it was soldered to rca jacks on the back. Undo the 2 screws holding the RCA plate. Undo the screws holding the arm board. Undo the screws underneath the edge (6 total if memory serves) and three on the top under the platter. The top should lift off enough to take the arm and RCA plate out. Remove the arm (you may have to melt the solder to get the rca plate through the hole in the armboard (I did)). The plastic under-chassis can get in the way of the leads but if you don't mind you can just melt away a narrow groove with a soldering iron. Attach the arm to the plate, thread in the leads and reverse the procedure for undoing the whole thing.
I didn't find that the weight difference was such that any adjustment was required. They're pretty old TT's anyway and I doubt that the suspension system works as it was when new.

True the RB300 doesn't have a VTA adjustment but you can use the specs again to compensate in making the thickness of the new armboard should you build one. Or use washers.

In my case, the sound was immediately noticable as being better. Much more detail and frequency extension. And the RB300 is such a pleasure to use after the fiddliness of the Sonus Arm. And it was a fun project.

I have a couple extra armboards I made of different woods if you'd like one. I had a few mishaps lining up the mounting crews and they work fine but one hole is a little ovoid due to being slightly off. I did one in maple, cherry and an extra in oak. I can't guarantee that the pivot center will be the exactly right as I didn't have, nor could I find a 23mm drill bit but it's pretty darn close. Close enough that my cartrige aligns quite well.