I think you'd be taking a chance that the buried portion of the standard might root causing sucker problems.
Were it mine, I'd dig under the roots with a spading fork and gently lift it up, pushing soil under the raised root ball to hold it at the level you want. I've done this with regular bushes with great results

Roses don't mind being planted deep. They're pretty much the only woody plant that won't die when the woody above-ground portion is buried. But, as karl says, it might start suckering. You could just add soil to the top, they won't die, but it might be best to lift them up and add the soil to the bottom.

To prevent them sinking again, make sure you have a mesh material over the drain holes. I use either window screen mesh or you can get special bonsai pot mesh made specifically for placing over drain holes. This keeps the potting soil from falling out the bottom. Either one works much better than anything else I've tried, like the traditional clay shard or rock. And, when you add the soil, dampen it and pack it down with your fist before putting the rose on top.

Karl/Reg,

Thanx for taking the time to post a response. I appreciate it!

Max