Time between prepare for grafting and the actual grafting

I will top a long hedge this winter and graft in another variety.  

Basically replacing an Elaeagnus (Pungens, Silverthorn, Ebbigini) with Gumi (Elaeagnus multiflora) that does not grow as tall.  Hoping to get Red Gem and Sweet Scarlet scions.
Elaeagnus is a not stop grower and will reach 12 feet if left untrimmed and it is a bear to trim.  These Gumi should only grow to 6 feet.

This is a long hedge (around 200 feet) so I want to take it out in stages and then cleft graft scions at bud break which leaves me a couple of days (I expect).

How long can I wait, from when I top all the bushes to graft the scions?  Would it help to leave a long trunk and cut it back just before grafting?

I got some advice from a nursery man who grows all kinds of alternate cash crop fruit trees.  He said that I could cut the bushes back down to about 2 feet and then right before grafting cut off 2 to 6 inches.  The weather must be between 70 to 80F and the root stock must be actively growing.

chop one year. Graft onto new year's growth.  Take scions during dormant winter period.  W/t graft in late winter/spring.
John S
PDX OR