If you don't buy or don't install the expansion, will your level 60 characters continue to earn paragon levels after the release of the Expansion on March 25th, or will they be locked at their (then) current level, unable to get more paragon levels until you install and level to 70?
You've not really answered the question -- I know experience is how paragon points are calculated, but paragon levels (pre-xpac) add stats. Will that continue after the expansion is released if you don't have it (the xpac) installed?
He answered your question, it's just that you phrased it wrong if you wanna know about stats... you were only talking about experience ("levels") in your first post ;-)
Even if you don't buy RoS and continue playing D3V, your game will be patched and get the new paragon 2.0, which means that all individual stats gained for all your characters through paragon will disappear. This includes the four core stats and all magic find. At the same time, all your characters will get your (account-wide) paragon points that you can then use to spend for the stats as you like.
When this will happen is what we don't know. Blizzard said that there will be a pre-expansion patch, but since the auction house will go offline only one week before RoS ships, they might just patch D3V on RoS release day. Or a week before. I doubt that the pre-expansion patch will come out several weeks before RoS - not only because it would allow for level 60 RoS loot to be traded on the auction house, but also because the game needs every single day of polish, it's still quite far from release in terms of balancing.
I doubt that the pre-expansion patch will come out several weeks before RoS - not only because it would allow for level 60 RoS loot to be traded on the auction house,
The only things that will be tradeable are rares, not legendaries when 2.0.1 hits live, so it wont be that big of an issue IMO if it hit early. Especially since rares can only roll 4 primary affixes in 2.0.1 instead of 6, which is the case with live servers now. The best items will be pre 2.0.1 for this reason (vit, main stat, all res, sockets is 4, you wouldn't be able to get armor or crit, or life%, or trifecta...ect ect), so I could see the pre expansion patch hitting live servers before the AH shutdown, although I agree with you it seems unlikely.
Back to the rephrased question by the OP: The stats that you got from paragon levels now will all disapear and you will instead get points you can allocate for different kinds of stats: main, offensive, defensive, and utility. New paragon levels won't be as strong as they are right now, but give a larger diversity of options and don't have a limit. The main thing you should be aware of is that all of your MF and GF from paragon levels will go away, regardless of whether or not you buy the expansion.