[Nottingham] Growing a RAID 1

[J I]

Hello everyone, hope you are keeping well.

I have an old RAID setup using drives from 2011 and whilst I've not had
issues, I am getting a bit nervous about them.

At the moment both 2TB drives are in mdadm RAID1 (/dev/md0), and LVM sits
on top of that, the actual FS is the unexciting Ext4:
#lsblk
...
sda                  8:0    0   1.8T  0 disk
└─md0                9:0    0   1.8T  0 raid1
 └─primary-varlib 253:0    0   1.8T  0 lvm   /var/lib
sdb                  8:16   0   1.8T  0 disk
└─md0                9:0    0   1.8T  0 raid1
 └─primary-varlib 253:0    0   1.8T  0 lvm   /var/lib
...
As you  can see they are not boot, just data really.

I want to replace them both with new 4TB Seagate IronWolfs

Can anyone point me at some instructions on how to do that? My search-fu is
failing me and I have a feeling it's going to be more complicated than just
replacing the drives one at a time with a rebuild (and then some magic to
grow things to the full 4TB).

Do I need to be overly concerned with /etc/fstab which seems to be using a
UUID:
/dev/disk/by-id/dm-uuid-LVM-47rf... /var/lib ext4 defaults 0 0

This is all on Ubuntu Server 22.04.

Cheers!

J.

Some more info if it helps:
#file -sL /dev/primary/varlib
/dev/primary/varlib: Linux rev 1.0 ext4 filesystem data...

#mdadm --misc --detail /dev/md0
/dev/md0:
          Version : 1.2
    Creation Time : Sun May 24 15:00:15 2020
       Raid Level : raid1
       Array Size : 1953382464 (1862.89 GiB 2000.26 GB)
    Used Dev Size : 1953382464 (1862.89 GiB 2000.26 GB)
     Raid Devices : 2
    Total Devices : 2
      Persistence : Superblock is persistent

    Intent Bitmap : Internal

      Update Time : Tue Jan 17 23:22:16 2023
            State : clean
   Active Devices : 2
  Working Devices : 2
   Failed Devices : 0
    Spare Devices : 0

Consistency Policy : bitmap

             Name : ubuntu-server:0
             UUID : 7df346bb:a3c7ee5a:aa7ea17b:09c1c352
           Events : 88901

   Number   Major   Minor   RaidDevice State
      0       8        0        0      active sync   /dev/sda
      1       8       16        1      active sync   /dev/sdb
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