When an AirPort is properly configured to extend the wireless range of another, it is automatically reconfigured into Bridge mode.
Each of AirPorts that you will use to extend the new 802.11ac AirPort Extreme would all need to be configured as such. Please check out this Apple Support article on how to extend AirPorts.
The key here is to understand that the extended network works in a wheel-like fashion with the base station being extended at the hub of the wheel and each of the extending ones on one of the spokes. This should help you plan the best placement of each base station for maximum performance. For more details on optimum placement, please see the following AirPort User Tip.
So I have re-set all my airports and extremes - ie held the reset button while powering on and waited for the amber light to flash.
Now my new tower extreme is up and running fine
1 of the airport extremes is discoverable and has a green light and appears in airport utility (it doesn't appear to have lost any of its identity though - same name as in the past).
2. my original airport extreme (it used to be the base station for my whole network) just reverts to flashing amber no matter what I do and is not working or discoverable. Is this because it conflicts in some way with the new tower base station that has taken over its role and settings (I chose the same wifi network name) - but it should be re-set should it so no conflict????
It appears that neither of your older AirPort have reset properly. Please check out the following Apple Support article on how to perform a "factory default" reset for each of them. Note: It may take several attempts to do so. In addition, to help facilitate the initial setup bring each of the base stations into the same room. This way you can be assured that they are all within range of each other and communicating.
Yes, I ended up re-checking the reset - then bringing all the airports into the room with my tower and my mac in order to make them discoverable. They showed up immediately. So now have the tower as the main base station beaming to two extremes in the upstairs wing of house and seems to work well. One other question I have is what to do with my "spare" airport expresses. I am thinking of plugging them in directly into 3 of my apple tvs with ethernet. Would that mean more efficient use of my internet. Does that mean that those airport expresses are then "tied" to the appletvs and don't dilute or disrupt my general extreme network? I assume it would make the apple tvs faster.
Lastly - thank you apple for the joys of appletv - it has freed me entirely from conventional and cable tv and the media available through podcasts, itunes, youtube, airplay, itunes sharing is now pretty diverse.
One other question I have is what to do with my "spare" airport expresses. I am thinking of plugging them in directly into 3 of my apple tvs with ethernet. Would that mean more efficient use of my internet. Does that mean that those airport expresses are then "tied" to the appletvs and don't dilute or disrupt my general extreme network? I assume it would make the apple tvs faster.
If by "older" you mean 802.11g versions of the Express, you will not be able to do this without significantly affecting bandwidth performance. However, if they are 802.11n models, then they can be reconfigured to perform as wireless Ethernet bridges. They would join the extended wireless network and their Ethernet ports would be enabled for the Apple TV. However, you may want to experiment as using them this way may not offer any performance advantages vs. just connecting the Apple TVs by wireless.
So another question. I currently have 1 new airport extreme as my incoming wifi station sending to 2 (1-2 year old) airport extremes (bridge mode) and an airport. I now have pretty good wifi throughout the house which has a lot of solid walls and bathrooms hence the need for many routers.
However I notice that 1 of the airport extremes is taking all the load - ie 3 of the apple tvs and various ipads etc are connecting to it automatically - lowering its performance to "good". Meanwhile the airport which is nearby is getting no connections and has "excellent" performance. Is there a way to determine which routers the apple tvs connect to? I've started turning them on in sequence to force a connection but it does not really seem to work.