On trying to upgrade from OEM to LEDE on a Linksys WRT54GL, I get the error incorrect image file after clicking upgrade on the OEM firmware upgrade page. The file I'm using is openwrt-brcm-2.4-squashfs.trx which is the file from the HW Finder.
[EDIT] It's the wrong file. WRT54GL users should refer to the WRT54G product page for specifics. The proper file, after some digging, is v10.03 for WRT54G. I upgraded to v4.30.18 of the OEM firmware first, but this shouldn't be necessary. Someone should try to install it using another version to verify (it won't brick).
OK, I just attempted to flash a Cisco Valet M10 with version 18.06. The 18.06 firmware image is too big (17.01 only has 200 KB remaining after flash anyway). The M10 (E1000) also has a 4 MB of flash. I had to SSH and use CLI to see the error.
I believe I'll run into a similar issue with the GL.
I have a WRTSL54GL (8 MB flash), version 18.06 installed with no issues. If this is the case, most 4 MB devices are no longer supported - non-LuCI builds might work (i.e. SNAPSHOT).
EDIT: My WRTSL54GS constantly crashes under WAN <> LAN traffic (I've only use these devices successfully for WiFi <> Wired client/bridging since Kamikaze), with or without offloading enabled (30/30 Mbps no offload, 62/62 Mbps with offload before crashing). It seems I may have to revert to 17.01.4 (which wasn't much better).
There is a reason why the dataentry shows 10.03.1. I'm pretty sure there was a discussion about those devices which lead to openwrt-brcm-2.4-squashfs.trx in the dataentry, but I don't have the time now to search for this.
Thanks for your edit of the first post.
I would recommend to use recent openwrt code because of many security issues fixed between your installed openwrt 10.03 and recent code.
You have to note, that because of the low amount of ram (16MB on WRT54G) you would probably have problems with luci. So the WRT54G with 16MB of RAM should install just the "snapshot" builds that dont contain luci. They work fine on 16MB RAM devices so far.
The WRT54GS that have 32MB of RAM. If you can fit Luci on the ROM, that it should work fine. Otherwise also just use the snapshot builds to have latest security fixes.