it is indeed a pretty interesting idea. I don't know about the feasibility of grafting into mature trees, compared to grafting into younger trees-- you may need to consider overall health and longevity, if these trees are old they will have a natural lifetime that will need to be considered. Likewise their overall health (or maybe external issues like water availability)-- have you determined why this orchard is for sale?
Another concern I would have is soil quality and contamination. Crete may not have this problem but where I live orchards and vineyards often have buildup of copper from bourdeaux mix, which is considered an organic anti-fungal, but it accumulates and can cause problems. But this is mostly a due diligence problem.
I did a quickie search about survival in grafting to mature trees and Perplexity said this:
Grafting new varieties onto mature citrus trees, known as topworking or reworking, is a realistic and established practice with success rates often exceeding 75-90% when done correctly. Healthy trees benefit from their established root systems, enabling rapid regrowth and fruit production in 3-5 years. However, success depends on proper technique, timing, and health management.
Success Factors
Techniques like bark grafting, wedge grafting, and cleft grafting work well on limbs under 150 mm in diameter, performed during late winter to spring when bark slips easily. Use disease-free scion wood from compatible varieties, such as oranges or mandarins on trifoliate rootstocks, to avoid incompatibility or virus issues like exocortis. Nurse branches left on the tree aid sap flow and protect grafts from sun and wind.
Risks and Challenges
Unhealthy or old trees yield poor results, and large pruning wounds near the ground risk heart rot or pathogen entry in citrus. Lemons and grapefruit reworking is less successful due to virus transmission. Post-graft care includes bagging grafts, monitoring pests, and adjusting irrigation to prevent failure.
Practical Outcomes
Mature scions can fruit within 1-3 years, with real-world examples showing multiple grafts taking on stumps and producing fruit in 18 months. Full production returns quickly, though professional help may be needed for large-scale efforts.
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I suspect you would do better keeping a part of the valencias and cutting/replacing with other species. You'll need to be very alert to water needs, I suppose. Might be able to intersperse with grapes, blackberries/raspberries, pomegranates, and maybe even nuts (not sure what kind works with your climate).
