The short answer that the architect must agree in writing to assigning a new architect or engineer of record and must agree to allow the new architect to take ownership of all work product and this is assuming that you can find a new architect that is willing to accept liability for plans that they did not fully prepare. You will also need to do this for any other engineering professionals that were part of the design team (structural engineer, civil engineer, geotech etc.) unless you can take over their contract from the old architect. This approach is the last resort.
Unless the property purchase agreement included an assignment of the architect's design contract that was signed by the architect, you have little leverage to compel them to perform other than throwing money at them to make it worth their while.
Option 1 (best option): I would highly recommend sitting down with them, getting a new contract in which they are contractually bound to you and throw enough additional money at them to get you to their front burner. This way you have leverage and if (when) there are issues during construction they are obligated to work towards a solution. This is the cleanest and likely the quickest solution.
Option 2: Hire a new designer, keep the original engineers (with a new contract) and have the new designer start over with enough design changes that the old architect does not sue you for copying their design. This will still be quicker than starting over, but you will still need to start from zero with architectural plans and plan check.
Option 3: Start over with an entirely new design team (if engineers will not play ball).
Option 4: Have your broker contact the seller's broker and get the seller to push the architect since they advertised that the plans were almost ready. This may help wrap up plan check but you will still need the architect and engineers to perform certain services during construction so you will need a contract with them for this portion of the project.
Option 5 (worst option in my opinion): Change designers and get the old architect to release the plans as discussed in the first paragraph. This option is fraught with challenges and lets both the new and old architect off of the hook for design errors. There are other potential issues nuances with this solution that are beyond the scope of this short post.
This is a difficult and frustrating situation and I hope you can resolve it quickly!