The Implementing Rules for the Basic Instrument Rating (BIR) has been published in the Official Journal of the European Union today.
Easier access for GA pilots to Instrument Flying Rules (IFR) flying is considered a high-priority measure that will improve the safety and utility of GA flying. The Basic Instrument Rating (BIR), introduced by Regulation (EU) 2020/359 and published in March 2020, is an amendment to Regulation (EU) No. 1178/2011. The new BIR requirements that will apply as of September 2021 introduce a qualification to fly in accordance with Instrument Flight Rules (IFR), but based on more proportionate requirements when compared to the traditional Instrument Rating (IR). Both privileges and competency-based training requirements in the BIR are tailored to the needs of GA pilots.
Commission Implementing Regulation (EU) 2020/359 of 4 March 2020 amending Regulation (EU) No 1178/2011
December 2020 update: The AMC and GM to Part FCL covering the BIR has now been published as well. More promotional material and information will follow before it starts in Q3 2021.
There is no minimum training requirement, so it is hard to see how any credit could be given. The BIR is truly "competency-based", so AFAIU you only need to train "to proficiency", i.e. be good enough to pass the skill test. A pilot who has been actively using his IR(R) will probably need little training towards module 1, but will need training for modules 2 and 3.