Here are the basics that should be on your website as you transition:
Add your new giving link
Explain the transition: have pastor or team sourcing the new giving software write content or an article explaining the vision and process
Make sure new recurring gifts cannot be created on the old platform, but that existing recurring gifts to your old platform can be cancelled easily
Check out these examples:
Demo Example
 This demo website has recommendations for layout and verbiage:
(click image to zoom in or enlarge)
Beltway Park Church Example
Their main giving page has a brief announcement about the change linking to a more detailed explanation.
(click image to zoom in or enlarge)
Other things to consider adding to the Giving section of your website: