diff --git a/docs/installation/upgrade.md b/docs/installation/upgrade.md
index 7e143046..9bf2279f 100644
--- a/docs/installation/upgrade.md
+++ b/docs/installation/upgrade.md
@@ -54,7 +54,12 @@ Steps:
 Then, for every host of the pool, starting with the pool master:
 
 3. Move all VMs off the host if your setup allows it, or turn them off.
-4. Follow the installation procedure on the [download page](https://xcp-ng.org/download/).
+4. Follow the installation procedure on the [installation page](../install-xcp-ng#start-the-host).
+
+:::warning
+You must boot the ISO in the firmware mode that was used for initial installation: UEFI or BIOS. Otherwise, the installer won't detect the existing installation and offer to upgrade.
+:::
+
 5. When offered the choice, choose to upgrade your existing XCP-ng installation.
 6. After the upgrade completed, reboot your host.
 7. [Install the updates](../../management/updates) that have been released after the installation ISO was created. They can fix bugs and/or security issues.
@@ -252,6 +257,10 @@ XCP-NG installation follows roughly the same workflow as a XenServer installatio
 
 Then boot on the ISO!
 
+:::warning
+You must boot the ISO in the firmware mode that was used for initial installation: UEFI or BIOS. Otherwise, the installer won't detect the existing XenServer installation and offer to upgrade.
+:::
+
 ![](https://xcp-ng.org/assets/img/screenshots/install1bis.png)
 
 Eventually, you will reach a screen offering to upgrade your XenServer to XCP-ng:
diff --git a/docs/troubleshooting/installation-upgrade.md b/docs/troubleshooting/installation-upgrade.md
index 2187f15b..b58c3e7d 100644
--- a/docs/troubleshooting/installation-upgrade.md
+++ b/docs/troubleshooting/installation-upgrade.md
@@ -29,6 +29,35 @@ When asking for help about installation errors, providing this file increases yo
 
 The target installation partition is mounted in `/tmp/root`.
 
+## The ISO installer does not offer to upgrade the existing install (XCP-ng or XenServer)
+
+For the installer to detect your current install (XCP-ng or XenServer), the ISO must be booted in the same firmware context.\
+The difference is how you pick the drive at boot. This applies to physical and virtual CD/DVD/USB drives.
+
+If your existing installation is in legacy BIOS mode, boot the ISO in BIOS mode.
+
+If your existing installation is in UEFI mode, boot the ISO in UEFI mode.
+
+In your server boot menu, you might see something like this: 
+
+```
+Please select boot device:
+---------------------------------
+UEFI: Virtual CDROM Device   <<<<<< This one is in UEFI mode
+Virtual CDROM Device         <<<<<< This one is the same device, in legacy BIOS mode
+...
+ other boot entries
+...
+---------------------------------
+```
+
+### How to check if a running install is using UEFI or legacy BIOS?
+
+On the host, run `efibootmgr`.
+
+- If you see `EFI variables are not supported on this system.` you're running on legacy BIOS.
+- If you see some EFI boot entries, you’re running on UEFI.
+
 ## Installation logs
 
 The installer writes in `/var/log/installer/`.