Is it ok to replace all 2-prong receptacles with GFCI ones without identifying head of circuit?
If you want, you can certainly put GFCI outlets everywhere. Just be sure to not use the LOAD terminals on any of them. Instead, use pigtails to connect both incoming wires and outgoing wires to the LINE terminals. If you have any outlets where multiple hot and/or neutral wires connected (any the tabs are not broken), do not use the LOAD terminals to reproduce that.
Having said that, it's not that hard to eliminate the need for many GFCI outlets. Start with an outlet in a room. With the power off, remove it from the wall and disconnect all of the wires (make sure you note how they were connected if the colors are not obvious) and cover them with wire nuts. Turn the power back on and see which outlets are now dead: these are all downstream and can be replaced with normal 3-prong outlets, labeled &quot;GFCI protected/no ground&quot;. After idntifying the incoming wires (with a voltmeter or non-contact tester), turn the power back off and replace the outlet you pulled with a GFCI outlet. Make sure to connect the incoming wires to LINE and outgoing to LOAD.
Now try another outlet, if it turns out that a new GFCI outlet is downstream, either make that an isolated GFCI or go back and replace the downstream GFCI with a normal 3-prong (labeled).
Don't forget to label all GFCI outlets &quot;no ground&quot;.
One warning: if you try to use an outlet tester with a GFCI trip button to test your wiring, such as

be aware that this will not work without a ground. You can get around this by using a long wire with one end connected to a ground (a water faucet my work) and the other end connected to the tab on a 2-3 prong outlet converter

and plug the tester into that.