How to convert eaton type BRD to type BR and is this a good idea?

First, make sure you are searching broadly. Many homeowners get stuck in the "big box store" rut and search only Lowes, Home Depot and Menards... failing to realize that family-owned lumber supplies, electrical supply houses, and hardware stores all sell breakers.

Look at your bus stab for notches. Or, Plug-on Neutral.

You cannot put tandem breakers anywhere except where it's allowed by the panel labeling. Fitting them where they aren't allowed generally means putting more breakers in the panel than it was designed, tested or approved for.

It is common for Eaton's label to not specify which spaces are allowed (some diagrams don't even correspond to the panel)... in those cases, they instruct you to look at the bus stab: a solid/square bus stab does not allow tandems/quadplexes, however, a notched bus stab does.

The reason is CTL, or Circuit Total Limiting. This was a NEC and UL requirement from 1966 to, I want to say, 2014 or 2017. CTL panels have notches in their bus stabs where tandems/quadplexes are allowed. CTL panels also have labeling indicating that you must use "CTL" breakers such as BD, BQ, or BQC.

Of course there were some pre-1966 BR panels still out in the field, which were entitled to use double-stuff breakers, but did not have stab notches. To support these panels only, Eaton continues to make the "BRD" breakers - however it is a Code violation to fit a BRD breaker in a CTL panel, because that would let you defeat the CTL limitations.

NEC recently dropped CTL as a requirement. That does NOT give you a free ride on any existing CTL panels; they have not been tested by UL for, and have not been approved for, double-stuffs in any space. Keep in mind that panel makers did not immediately change their designs and immediately get UL approval: so CTL still applies to older designs bought today.

However, if you buy a new design panel which has been UL-certified to be non-CTL, then you can (as always) use any breaker listed in the panel's labeling. ThreePhaseEel, who knows 10 times more about panels than I pretend to know, says that all Eaton "Plug-on Neutral" designs are post-CTL and are approved for double-stuffs in any space, including BRD type.

All that to say... if you have a CTL panel and the breaker space is allowed to use a BD breaker... I don't see a problem with temporarily using a BRD breaker. You'll need to correct it before next inspection (typically at house sale time). And make sure no maintainer moves it into a non-tandem space. It's a violation but a statutory one.