The point that is made, but not elaborated on so much as is the difference between standard and high performance steel loadings, is that modified (half) choke should be the maximum constriction if using steel.

Choking is the main reason why many older shotguns are not suitable for steel shot. The older guns had fixed chokes and most often were tighter choked e.g. full and full or maybe improved modified (3/4) and full. This is the issue and it would likely be a little expensive to have the chokes opened up to use steel, likely cheaper to purchase another gun with screw in chokes.

Not all 'modern' shotguns that can shoot both standard or high performance have the fleur-de-lis proof mark. Browning, Miroku and Winchester shotguns made by Miroku with invector type chokes can safely use high performance shot even without the fleur-de-lis proofing.

The below taken from the Browning website;

"All hunting or sporting shotguns (over-and-under or semi-auto guns) by Browning, Winchester and Miroku fitted with the original chokes (Invector, Steel Invector Plus, Stainless steel Invector Plus, Teague, Briley, Midas, Diamond, Signature chokes) can fire "high performance" steel shot cartridges.

Tested at the Ličge test bench (high performance 1370 bars) in accordance with European law on the use of steel shot"

I've used high performance steel in my 1984 Miroku O/U gun for years now without any ill effects. I usually use the skeet or improved cylinder (1/4) invector chokes depending what I'm hunting.