What’s the process to transfer from IRR to ARC with a reclassification?

You will go to reclass school possibly L. Wood ft. Polk or there is a place in California also

Tranining is easy

You will attend 12N RECLASS. It's two 2 week phases. You may have the opportunity to complete back to back. Your Training NCO will be able to provide potential training dates for you. The course is held in multiple locations depending on the units quotas. I completed phase 1 in FT Hunter Ligget and phase 2 at FT Leonard Wood. Great course. Welcome to the Army Reserve.

I have not heard of a Reserve crash course for Engineers. If you are coming into a regular Reserve Engineer company- those line units are typically "can train" units & probably have seats to reclassify into different Engineer MOSs.

Since you won't really be qualified until you go to the course, the unit will want to send you to the course: I did look in the ATRRS Course Catalog & it looks like the USAR does have some AIT-type 12N courses at Fort Hunter Liggett, CA; with Phase 1 & Phase 2 Reserve courses. These both 4-5 weeks long each & you could probably knock these out in the next year.

Your unit training NCO would be the best person to contact regarding seat availability. I don't think the recruiters handle reclass-type schools (only the initial entry Soldiers) - but you can check with them to be sure.

I am sure the Reserves has a reclass course, it is the same POI as active just has longer days scheduled to compete the material in a shorter span, in some cases they may have several phases. I know I reclassed to 92L and phase 1 was 14 days, phase 2 was 15 days, all but one soldier in the class took both phases back to back. But I would say even if it is a reserve course there is a good chance it will be at Leonardwood.