I wanted to know can we associate licensing for the managed package after the security review ?
Yes, you'll be given directions for how to install the LMA and associate your package to the LMA. This will enable the ability to log in as a subscriber and manage license count, trial mode, etc.
Can we change the licensing model at a later stage from site wide to per user?
Yes, you can change your pricing model if you need to. The amount that you'll pay to salesforce.com is based on a percentage of the fees you collect for your licenses, no matter how they're licensed.
How would current subscribers get affected when you change the licensing model?
Each subscriber can be licensed either Site-wide or by a number of Licenses. Changing your listing has no effect on current subscribers. You can change Site mode or license count at any time that you want per subscriber. Of course, assuming you're signing contracts, make sure you honor them.
We are unsure about pricing now? Is that something that can be changed later along with the licensing model
Yes, you can choose to alter your fees at any time. You can offer discounts for volume purchases or extended contracts, etc. Again, you only pay a percentage of the fees you collect in regards to your licenses. I've seen pricing matrices, flat rates per license per org, etc. It's really all up to your organization to determine the most appropriate license fees for their application.