The question is, can we now proceed with a solicitation under the approval authority of the original Acquisition Strategy/Plan that included the Small Business Set-Aside or do we now have to prepare a new separate Acquisition Strategy/Plan to only cover the SB Set-Aside?
Since it has been 4 years since your acquisition strategy/plan was approved, you may want to do additional market research to determine if it is still valid.
Typically, if your plan said you would have a multiple award contract for both unrestricted and set-asided it would indicate a partial set-aside (19.502-4) or a small business reserve (FAR 19.503). It appears from your question that was not the case. You could have written the original solicitation which you made awards from to also be a partial set-aside or reserves and make both unrestricted and small business awards. Please work with your small business specialist, counsel, and contracting officer to determine your way forward.