Generally, removing a couple of old canes will cause basal breaks to occur. You can best do this after the spring flush in order to maximize the spring flush off the old wood. During the flush you may observe that some of the old basals are more productive than others. New basals that follow in summer would then be ready to bloom heavily the following spring. Your plant is old enough that such regenerative pruning would normally be recommended.

I haven't grown DJ, but others can report their experience with regenerative pruning. There may be some varieties where it isn't needed or doesn't work.

I very rarely hard prune him in zone 5. The growth that does not die back is precious in my short growing season. Mine is also on a trellis and I see most old wood in the center. In the spring I bring this down to a foot or so. I usually have lots of basals but I attribute this to heavy feeding and not my pruning habits. He is one of my favorites.

Bump...I was researching this over the summer when I noticed how fat my Don Juan's main canes were getting. I wrap my DJ in winter to help save the canes from dieback.

My DJ is probably 4 to 5 years old, and grew back from total dieback after his second winter. I now have 1.5"+ canes and his longest canes are 12' after this summer.
I think KingCobb and I want to know if hard pruning old "woody" canes out is necessary as long as they are still healthy...

I did let a couple basal canes fatten up and grow long last summer as insurance against damage to my main monster canes over the winter.

Do we have to?! What is the benefit?

"I did let a couple of new basals. . . grow long"

I hope this doesn't mean you have been removing other new basals.

In zone 5, I would not remove old primary canes of DJ as long as they are producing decent bloom in spring. The old woody canes are more likely to survive winter than green, one-year-old canes. DJ is not a very hardy climber.

The reason for removing canes that are 4+ years old is that, on many or most varieties, old canes become less productive. Removing them in spring encourages new basal canes and keeps the plant young.

I get several basal canes each spring and remove the weaker ones. The pic is from June 2012, and the mild 2011/12 winter and a burlap wrap meant that this rose doubled in mass this past summer.

If you see the thick bare cane curving up and back in on the left, it has a thick lateral that pushes up to the right--these two canes (really just one) and another thick basal cane, which originates on the left and crosses behind this "Y" cane, supply 75% of the mass of the rose.

I have yet to hard prune any canes larger than a pencil. As you may notice, I let several small basal canes/laterals help fill in the void of buds at the bottom of the woody main canes. These are pruned back slightly after flowering.

Without looking at summer 2013 photos (my phone's dead), I let a very thick basal grow to about 4' on the left side I think from behind the trellis before bending back in toward the structure, and let a similar basal grow from the right side...it was oriented more forward-facing before it was trained back to the structure.

These main canes zig-zag across/through/sometimes behind the 6' tall trellis and this summer I let laterals grow in a sort of fan from the top of the trellis to attach to the deck railing above.

I guess the question that lingers from my earlier research is: can a cane get "too thick"? That is, is there a point when the very thickness of the cane inhibits nutrient uptake or risks cracking under stress of wind/cold/its own weight?

I let those new basals grow anticipating that 2-3 big-daddy canes will crack or fail in some way in the next few seasons, and I want to have some mass ready to camouflage the loss. I pruned the other basal shoots to encourage growth in the upper laterals. Does that make sense?

I can't say that I've noticed fewer buds on laterals from the larger canes at any height.

Such a beautiful rose. And I hang my climbers, too!