Now I think back to the first plan, but now the question is when. I would not think summer is optimal. The fact yours survived must mean that yews are tough, but it doesn't seem like the best time. Some sites said spring, some said fall, and some said winter, so I guess that means almost anytime will do.
Maybe tomorrow.
In my garden, yews have a serious push of growth in spring. Then in summer I might get a second push. If I trim and pluck the branches in the mid of summer, I get bud-setting all over and sometimes breaking befor fall, but only branch ends that were not trimmed continue to show real growth.
In my street I have seen a yew been completely bared one late summer. An empty trunk. Nothing visibly happening. Untill about may the next year, when the tree started pushing and grew easily half a metre in one push. Mind you, this was a big tree that became small.
To me, it makes sense to align with the big spring push. Make sure well before bud-break you have cut. Give the tree time to wake dormant buds, trigger them into developing so near regular bud break they can push.
I think I mentioned it before: Be certain your tree really is healthy & well rooted if you take such steps.