Does the motherboard 766715-501 have a replaceable CPU.

CPUs are soldered in these - Intel and AMD. Most laptops made after 2013 are soldered since the PGA option has fallen out of favor.

In addition to that, processors like the mobile Xeon are reserved for high end multi-thousand-dollar mobile workstations like the Zbook, Precision and P series; not entry level models like the 15-p. These things need a Xeon capable PCH and are often as thick as the old Pentium 4 laptops from the WinXP era and have the cooling to match due to the TDP of the CPUs and GPUs they use.
Yes, the performance of a Zbook with a Xeon/AMD FirePro (ISV certified Radeon) is a huge step in the high end, but you’re gonna pay for it new, and to a degree used. Personally, I’d LOVE to take home a 4K (95% sRGB or DCI-P3) Xeon/FirePro Zbook 15/17 (I’d be able to go either way, but 17>15. It's a brick no matter what, so go all in and go Zbook 17) at EliteBook pricing too, but it isn’t easy — it’s gonna have a problem like an unknown BIOS password at those prices.