Hi Counterterror, you can't keep the old hardware anyway, except for the case, memory and possibly the power supply.
Finally, the question is what you are willing to pay, for a start I would currently rather go for a B660 board with Ddr4 and a 12400(f). However, it is difficult to take this with you, as you don't really know what connections are available for another board/HW. With the housing of such ready-made PCs it is also not always a matter of course that normal ATX or other sizes of mainboards fit in. You will also need a new CPU cooling system, the one from the 4820k will definitely not fit (at least not without adaptation), since the 12th Intel generation uses socket 1700. Best regards