Difficulties upgrading a frugal install

[novax]

Hello

On my PC I use Puppy 5.2.0 (frugal install in a FAT32 partition), other distros and Windows Xp; the bootloader is Grub4Dos, installed in sda MBR using Grub4DosConfig.
Yesterday I tried to update Puppy to the 5.2.5 release. For the first time I decided to upgrade (instead of making a clean installation, which is what I did in the past).
I simply extracted the files initrd.gz vmlinuz and lupu_525.sfs from the iso and copied them into the directory of Puppy 5.2.0, in place of the old ones.
When I tried to boot Puppy, I only got this error message:

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Booting Lupu 520 (sda2/puppy)
(hd0,1)
[Linux-bzImage, setup=0x3000, size=0x21d050]
[Linux-initrd @ 0x5fe1b000, 0x1d0982 bytes]
No setup signature found

I made an other attempt: I created a new directory (sda2/puppy525), copied into it the three files extracted from the iso and added a new entry in menu.lst of Grub4Dos, copying the Puppy 5.2.0 entry, with the necessary changes due to the different path.

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title Lupu 525 (sda2/puppy525)
  find --set-root --ignore-floppies /puppy525/initrd.gz
  kernel /puppy525/vmlinuz  pmedia=atahd psubdir=puppy525 pfix=fsck
  initrd /puppy525/initrd.gz

When I tried to boot Puppy, I got the same error message.

The last attempt: I launched the live iso of Puppy 5.2.5 and used Grub4DosConfig 1.7 to install Grub4Dos into a floppy. Grub4DosConfig correctly recognized Puppy 5.2.5 and his new path (sda2/puppy525) but, when I tried to boot Puppy 5.2.5 I got the same error message.

Which is the correct procedure to upgrade?

Thanks for the help