Old router has Gargoyle v1.6.1. How to safely update to LEDE?

Anybody know what the proper procedure is to get this router to LEDE stable? I'm assuming it's not safe to try to update directly from Gargoyle v1.6.1 to LEDE 17.01.

Gargoyle v1.6.1 is based on OpenWrt Attitude Adjustment branch, Gargoyle revision 87ce626, OpenWrt revision r40423. Target is ar71xx (Netgear WNDR3800 router).

I don't know if Attitude Adjustment has a sysupgrade command; if it does, it's as easy as doing a sysupgrade -n /tmp/$sysupgrade_image on the command line. If not, then get the router in recovery mode and TFTP the LEDE 17.01 factory image.