I always recommend buying something based on the features you get and the price you pay. If C7.5 offers the features you need or want at a price that makes it a good value for you then buy it. It would not be economically wise to buy a product if it doesn’t have the features you want or need in hopes of getting a free upgrade to another product that also may not have the features that you want or need. By using this “is it worth my money” logic to decide on buying products, you can avoid that sad feeling of knowing you missed out on a free upgrade because you can just remind yourself that you were happy with the price when you decided to pay it.
If getting the free grace period update is the most important thing, then you could buy it now and wait to activate it until after the grace period starts.
In summary: If you want C7.5, buy it… If you don’t, but are aiming for C8, the question I would ask is, why? We don’t even know what features it will have or when it would be available.