The SHIFT5.1, SHIFT5.2 and SHIFT5.3 were backward compatible phones, so you could upgrade the PCB of the SHIFT5.1 with the PCB of the SHIFT5.3.
As someone who ordered the Librem 5 v1, I’m hoping that the PCB in the Librem 5 v2 will backward compatible. However, I think it likely that Purism will try to make the Librem 5 v.2 thinner and lighter, which makes it unlikely that its PCB will be backward compatible. Purism mentioned in one of its posts about Birch that later batches of the phone would likely be thinner, but that didn’t happen, so I assume that Purism will do that in the Librem 5 v2.
Of course, my real hope is that Purism will make its next phone based on the RK3588, which makes backward compatibility even less likely. The PinePhone 2 based on the RK3566 is going to have much better specs than the Librem 5 v2 based on the i.MX 8M Plus.