Generally, training a paniculata into a standard or tree form begins quite early, when the plant is little more than a tall seedling - I have no idea how successful you will be with a plant that already has some maturity and size to it, but it could be worth a try. Or you may want to try some from cuttings of your existing shrubs - this may be a more successful approach. You want to select a stong stem that is growing vertically or nearly so and as straight as possible and remove all the others. Staking is probably a good idea until the branch/leader has developed some girth and structure and becomes self-suporting and that could take time. You want to remove any lateral shoots that may emerge along its length except for any terminal growth. Terminal growth should be allowed to develop unchecked as this will be needed for the photosynthesis required for the plant to convert energy and nutrients into food. Gradually, the lowest terminal growth is removed until the stem has extended to the desired height, forming your trunk. The canopy can then be shaped as desired. This is a process that takes a number of years to achieve. Commercially grown tree-form hydrangeas are at least 5-6 years old, often older depending on size and the training and pruning is pretty consistent throughout this time period. That is one reason why tree hydrangeas typically have some hefty price tags associated with them compared to shrubby ones - you are paying for the additional time it takes to get this plant to a marketable size and for all the considerable hands-on effort of the grower. An alternate method that may be more appropriate for already established plants such as yours would be to select several (3 or other uneven numbers are suggested) of the strongest and most upward growing stems and focus your attention on them. Remove all the other stems at ground level and again, continue to remove all lateral shoots that emerge from the framework stems until you have achieved the height you want. You won't exactly have a tree per se with this method but you will have most likely get a taller shrub with a much more tree-like appearance. Again, I'd suggest staking with this method as well as the flowerheads will weigh down the branches until they reach a diameter and woodiness to be self-supporting.

Hello, I have a pink diamond tree also. I planted it either in 05 or 06 also. It is my only hydrangea tree and is about 6.5-7 feet tall. I am far from an expert but I will chip in my 2 cents. Either early spring or fall would be my best guess in when to transplant it. Hopefully someone else can tell you which of the two times would be the best. I try to never transplant shrubs/trees unless I absolutely have to. I bought mine from a local nursery and was in something like an 8 gallon pot or bigger. No real noticeable height has been gained either for me which is fine for me. Maybe has gained a foot or so at best across the years and mostly I would say from the head of it. I think once they train it to be a single lead and cut it to branch out. I dont think the trunk of the tree will get much taller in maturity. Only the head will continue to get taller/wider, but i think is limited by being a single lead in tree form. I think it would take a long time to train a single lead to be 10 feet or more tall and the growers want to cash out long before that...lol. But again I am no expert in it and I am happy with the height of mine in my situation. I see many people around my area with limelight trees that flop and need staked and seem to not like being in tree form. The pink diamond has never needed staked and seems like a very sturdy grower in tree form. Havent got to see a quickfire in tree form, but have them in shrub form. How fast they grow and well branched, my guess would be they will be another winner in tree form.

Pink Diamond IME has a mix of sterile (large) flowers and fertile (small bud-like) flowers. I don't know if the balance between the two is variable year to year or under different conditions. The polinators like the fertile flowers, and I like the looser look this gives the flowerheads, but you may not. Pinky Winky, Limelight, and Vanilla Strawberry have many more of the sterile flowers so their flowerheads are densely packed and heavier looking. See link below for photos of Pink Diamond flowers. Do a similar web search for photos of the H. paniculatas listed above to see how they differ in flower density, color, and flowerhead shape. Here is a link that might be useful: Pink Diamond flowers

I had to move mine just recently due to construction and don't expect a lot from it this year due to transplant shock. But I found a web photo of what mine usually looks like. It is planted in mostly sun and does great, though the blooms will last longer if you give it evening shade or dappled shade. It is more compact than my other paniculatas and blooms toward the middle/end of July. Soil won't really make a difference, just make sure you have good drainage. You will love this hydrangea!