How to change /boot disk?

[Chris Green]

My recent move to a (not seen by the BIOS) PCIe SSD seems to be fully
successful now, thanks to everyone here who helped me getting this
working.

Currently /boot is on a 500Gb SATA hard disk which is otherwise
completely redundant so I'd quite like to move /boot to the 120Gb SATA
SSD which originally had the OS on it.

So what's the easiest/simplest way to move /boot from one SATA drive
to another?  In my case I want to move it from /dev/sda1 to /dev/sdb1.
There's nothing on either sda1 or sdb1 that needs to be preserved.

Just to be a bit clearer here are the relevant bits from 'df' :-

    Filesystem     Type 1M-blocks   Used  Avail Use% Mounted on
    /dev/nvme0n1p2 ext4     48174  13927  31733  31% /
    /dev/nvme0n1p3 ext4    896193 273727 576874  33% /home
    /dev/sda1      ext4    468427    243 444322   1% /boot
    /dev/sdb1      ext4    112515  18036  88742  17% /1904
    /dev/sdc1      ext4    938772 168648 722415  19% /bak

I want to put /boot where /1904 is and then I can remove sda1. (/1904
was the old OS installation that I mounted for reference while I
configured my new xubuntu 19.10 installation on /dev/nvme0n1p2).

-- 
Chris Green