Traditional ISV conversion to become a pure SaaS vendor should carefully plan its application deployment strategy. Below you can find the steps list how to start suggested by David Linthicum. By learning the PaaS Market and selecting relevant vendors traditional ISV will present a fast go-to-market and eventually a smoother conversion to a SaaS vendor –

1. Understand the “as is” state of things: create an inventory of tools that currently exist and the skills of those who leverage those tools
2. Create your requirements: You need to define a solid list of features and functions that you’ll need your PaaS providers to support.
3. Consider your options: Look at the number of PaaS offerings out there and analyzing them as to their ability to meet the requirements defined in the previous step.
4. Consider performance, governance ans Security

Here are some important criteria for ISVs to consider in evaluating PaaS provider:

1. Moving:  When selecting a PaaS solution, the lesson is that you should always look for the option, if only in theory, to move to another provider without having to completely rewrite your application code. In order to decrease the lock-in risks, you must think carefully about your application business logic when implementing so when conversion is needed you will be able to use the data and the app structure from one platform to re-build the apps in another. Learn More about PaaS Lock-in
2. Development: Some PaaS platforms only offer part of the stack for you .Check out the enabled components so the solution will fit your solution needs including the presentation layer, business logic, security, database and web services.
3. Standard language: In order to decrease lock-in risks you will want to examine the PaaS language as Many PaaS solution are based on proprietary. (e.g., Apex, the proprietary language for SalesForce).Support for development tools and the development platform & languages supported by the PaaS platform is crucial in deciding the right one for your business.  It becomes more relevant when a vendor is able to support cross sets of platform and languages (e.g VM Cloud foundry support open source code) in its PaaS offering.
4. Integration: The integration with other services available from the vendor, provides the enterprises an option of building complex applications with ready to consume standard cross cutting services like email, search, content management, BI soutions and more.
5. Development and Testing Environments: Development of application in PaaS delivery model asks for offline development tools. Also, as part of the release cycles the deployment process Smart PaaS providers will have the capability of providing a staging environment which is close to the production.

Check out this list of vendors.
Do you still have a lack of knowledge with basic market definitions? Check I Am OnDemand Terminology Page