You must first describe the blooms on the old plants. Are they round mopheads that turn pink or blue? Round mopheads that stay greenish white? Panicle shaped creamy white? Some varieties can handle, even like, full sun. There are also some types of viburnum that mimic the look of hydrangeas.

Endless Summer is a macrophylla type that may not tolorate full sun and certainly won't tolerate full sun coupled with neglect. They will need frequent watering. Also, keep in mind that it is NEW and therefore doesn't have established roots capable of delivering prolonged moisture to the plant. Having said that, let me also say that all macrophyllas, established or not, can wilt in the heat of the day only to rebound after they cool off and no amount of water will change that. They will not however, rebound from sunburned blooms and leaves. That is why it is best to plant this type in morning sun and afternoon shade or all day dappled sun/shade. I think you are in for an uphill battle if you plant the ES in this spot.....yg

yellowgirl, thank you so much for the reply. I am not sure how to answer the (ver sensible) questions you pose, I don't recall the blooms well enough. Thank you for the info, it is very helpful! Jennifer

Those large plants may have been started in shade and then the trees chopped down, or some other change occurred after they were well established. YG's answer hinted at that, but I thought I'd lay out that possiblity. Do you see them in someone's yard that you know well enough to ask them about it? I often check out advice from places away from here, by asking about it on my regional forum. So, if you can get local advice about them, that would be unbeatable.