CRM 2015 Update 1 Introduces New form engine
There are changes in the page construction, frames handling, data loads, and how scripts are managed (custom scripts will be loaded in separate frames so that the main frame can be retained and reused for subsequent records).
Existing supported customizations, such as Client API, are still supported and so existing scripts should not require any changes.
Microsoft announced, that the unsupported customizations will likely break with the improved form engine. Any customizations should be tested after the Update 1 will be applied.