Can my old overgrown shrub be cut down and rejunivated?

Not sure - my book says pruning is not necessary, but branches can be thinned out in Spring. If its overgrown and you don't like the look of it, I'd chance it and prune it back in the Spring, say March or April. But you'll probably lose the flowers next year if you do.

if its very mature then reduce a third of it this year then the next third next year and then the rest the year after. that way if it doesnt like being runed you wont kill it. obviously try and do it so it looks balanced.
welcome to GoY too.