Sony and Microsoft are both considering introducing new versions of their consoles, possibly as early as later this year. Microsoft's Phil Spencer even hinted at this in a talk, as reported by Polygon. The rumors are that these machines would not represent a full new generation of consoles (which usually means a new architecture), but instead they would be a speed bump for better graphics performance.
Of course there are many questions, starting with "Why?" There's an obvious answer from the perspective of Sony, which is still in the business of selling TVs: The PlayStation 4 cannot produce 4K output, and increasingly 4K TVs are taking over in sales. Additionally, a PS4.5 (which some are calling "PS4K") could potentially build in the additional graphics box that will be coming with the PlayStation VR.
There's precedent for this sort of speed bump -- Nintendo's done that with the New 3DS, for instance. Of course, fans instantly want to know if there will be some sort of upgrade path for those who bought the original console. That seems unlikely, though certainly GameStop would probably offer a trade-in value for your old PS4 or Xbox One.
One thing that's quite different with these consoles is that they use standard PC architecture in many ways, so dropping in a new CPU and a new GPU wouldn't necessarily be all that difficult. Given how component prices have dropped, we might not even see much of a price increase. It's likely that Sony and Microsoft would keep their current consoles around at a $299 price point, and then charge $399 for the speedier version.
As far as compatibility goes, there's no reason that these console would have any issues running current software. Would developers conceivably build new titles that would require the added horsepower? Sure, but supporting the older consoles would be easy enough that it should happen in almost all cases. Games would probably just detect what hardware they were running on and change graphics modes automatically, or give you a couple of options.
These new consoles would also likely be able to read the new 4K/Ultra HD Blu-Ray discs that will be coming out. Otherwise the consoles would probably look much the same, except for some obvious visual way to distinguish them from the original versions.
No doubt consumers would be annoyed to some extent if more powerful versions of consoles they already owned were introduced. Some sort of trade-up program, even if it were done through a third party like GameStop, would help mitigate this backlash. Sure, people could sell their old consoles and buy a new one themselves, but it would be better if Sony and Microsoft sweetened the deal somehow; a low-cost way to do that would be to offer some free games for trading up
While still only rumors, this sounds like a good idea. This would extend the lifespan of existing consoles (and the sales lifespan of existing software), and keep up with the relentless advance of PC graphics, and provide more horsepower for VR and AR. For Sony, it helps sell 4K TVs and makes the PlayStation VR more attractive. For Microsoft, it might make the Xbox One more competitive graphics-wise with the PS4 (or rather, the PS 4.5), and perhaps provide enough horsepower for the Xbox One to drive an Oculus Rift. That would be a very smart move on Microsoft's part, even if it meant losing some money on each console sold.
This is a chance for Microsoft in particular to reset the console race and to actually move ahead in the VR race. Microsoft has had a huge advantage over Sony that hasn't been used so far -- Microsoft's huge pile of cash in the bank (well over $60 billion), compared to Sony still struggling to turn a profit. Microsoft could provide more horsepower than Sony at the same price, even if the company had to lose $50 or $100 per console. Say that's over 20 million consoles, by which time the component cost would have fallen enough to make much of that differential disappear. So it costs Microsoft perhaps $2 billion to catch up to Sony in terms of installed base; given how much more software the company could sell that seems like a reasonable deal.