I have an original Pi Model B which has been running Wheezy all this time, connecting to WiFi with a USB dongle (RTL5370) which I'm tryng to upgrade to Stretch.
I downloaded the latest Stretch (with desktop but without recommended software) and copied it to a fresh SDCard. To /boot I wrote a file ssh and a wpa_supplicant.conf file containing a copy of the wpa_supplicant.conf from my Wheezy installation.
I booted it up but it didn't connect to WiFi so I had to pull the plug. So I connected a screen and keyboard/mouse to see what was going on, and plugged in an ethernet cable. On the next boot I got a desktop and ssh login but shortly after I got segmentation faults to anything I did. I reseated the SDCard and rebooted, and this time it seemed stable but still not connected to WiFi.
lsusb shows the dongle is being recognised.
lsmod shows the rt2800usb module loaded.
ifconfig shows wlan0 with a credible MAC address but not connected.
/etc/wpa_supplican/wpa_supplicant.conf contains what it should, in fact identical to the Wheezy vertion.
/etc/network/interfaces just contains a pointer to /etc/network/interfaces.d, which is empty.
/etc/dhcpcd.conf has no mention of wlan0 and only commented lines relating to eth0.