Plugins:
In the 32bit version the old plugins from v1 should still work, the API has not really changed.
In the 64bit version, you will need plugins compiled for 64bit (the 32bit plugins can be installed, they just do not work).
In the past, when a plugin was missing, but the XMP had settings for it,
changing the settings resulted in nuking the settings for the plugin.
AS2 will not remove settings for plugins in the XMP if it does not have the plugin installed.
Noise reduction:
NoiseNinja is gone (due to Picturecode not renewing the contract with Corel,
guess they want to push PictureNinja

It is really completely gone.
The new Athentech Noise reduction seems quite capable, but with less control.
In contrast to the plugin settings though, NN settings will be removed when changing
the settings of a file with AS2.
64bit vs. 32bit:
You really do not need to use the 64bit version. AS2 does not gain anything significant in my experience.
On 64 bit Linux systems you should be able to install the 32 bit version after installing
some prerequisites without problems (works on my 64bit Ubuntu 14.04 systems).
Plugin users should at least wait until the plugins they use are available in 64bit before installing the 64bit version.
Prerequisites
Prerequisites can be a bit of a pain. On Ubuntu, the software center seems to be able to take care of them (most other tools can either install from a file or resolve dependencies but not both).
OpenCL:
OpenCL can lead to strange renderings. In AS1 and AS2 alike.
But, at least on my systems, the standard setting worked in
AS1 without much problems.
In AS2 I have to set OpenCL to a lower setting to avoid those strange artifacts.
Parallel install:
You can install AS1 and AS2 in parallel. You just can not run it in parallel.
Make sure you use different user home directories. Copy your old batch settings etc to the new one.
You can also install AS2 32bit and 64 bit in parallel.
(Well, RPM based parallel installs do not work, but fiddling wit the RPM spec file can make it work for the curious)
cheers
afx