Comparing Empire VFRL-18 and VFRL-18-10, the former is 26k BTU while the latter is 10k BTU. The look and price are identical. My question is, can I buy the 26k BTU unit to use in my living room, but if I decide to move it to the bedroom later, I can I step it down to 10k BTU to make it code compliant? In other words, how's the difference in BTU's physically implemented? Is it some part that can be swapped out?
It appears that the burner length is the primary difference, but that's not to say that there couldn't be other differences in the valving, etc. You'll probably want to contact the manufacturer if a company representative doesn't appear in this conversation.
A unique 18-inch (10,000 Btu) Millivolt Loft burner provides heat and ambience for bedroom fireplaces. Check your local codes for applicable restrictions.
The 24-inch Multi-sided Loft Burner (36,000 Btu) is ideal for Peninsula and See-Through fireboxes. ...
While it might be possible the two burner tubes themselves are the real difference and might be interchangeable allowing a 10K burner to swap into the 26K housing.
The housing would still be stamped/marked for 26K BTU. With the strick code for your area, if inspected would be subject to inspectors determination of BTU value. You would likely have to produce documented facts that the change was made and meets the 10K requirement as well as certified safe for use.
I would not risk possible penalties vs. cost of the second unit. Or if code permits the 10K unit in the living room then look at buying it, worst case then would be having to buy a 26K BTU unit if the 10K was not adequate for the living rooms heat.