I'm personally not aware of anything breaking moving from CF 8 to CF 10/11 (but you'd have to test your application to be sure). You can no longer purchase CF 10 directly, but you can purchase CF 11 and request a downgrade serial number to run CF 10 if you choose. Since there hasn't even been a single update released to fix bugs in CF 11, I would be very hesitant to deploy a production application on it. CF 10 has been pretty stable for me, at least with all the available updates installed.
As for the lifecycle (I'm assuming you mean support), you can find that information here: https://www.adobe.com/support/products/enterprise/eol/eol_matrix.html#63. The core support for CF 10 runs through the middle of May 2017, with extended support ending mid-May 2019. For CF 11, core support runs through the end of April 2019 and extended support through the end of April 2021.
We encountered a few minor compatibility issues between CF8 and CF10, but most were related to more complex/non-standard tag use within the applications. Like in our apps we prefer to control the client side cookie storage using CFHEADER and by default, CFHEADER blocks CFID and CFTOKEN as variable names. I vaguely remember one other issue that was simple to fix but I can't remember off the top of my head. Between CF10 and CF11 we have not encountered any compatibility issues. The speed and security improvements are worth the upgrade. Good luck.
We have don migration CF8->CF10 last year and some modification where needed (I can't remember exactly).
For 10->11 the following have to be consider: Changes in ColdFusion - ColdFusion English Documentation - Adobe Learning Resources