Action add to comment ThisRow_Before column value and ThisRow_After

I have reviewed the release notes here

but I am uncertain how to make an action work that would add to my previous Remarks column the value of a column in its before and after state.

I am using this expression:
CONCATENATE([_THISROW_BEFORE].[Remarks], " ", "Reassigned from ", [_THISROW_BEFORE].[Tech], " to ", [_THISROW_AFTER].[Tech], " on ", TODAY(), ".")

It seems like within the action expression to change the value of a column [_THISROW_BEFORE] or after is not even recognized, but when I rewrote the process in a bot, the expression was valid but the before [Tech] and after [Tech] values are the same.

Any help would be appreciated.

Not sure if this is your issue, but you should just use [Tech] instead of [_THISROW_AFTER].[Tech].

Thanks for the suggestion. I tested that and unfortunately that did not make any difference.