Pay Now payment overwrites the contribution

Currently when we do a pay now contribution civicrm_contribution.contribution_page_id and civicrm_contribution.source is updated i.e the configured page for the later pay now payment overwrites the contribution info derived from the original creation of the pay later contribution. This should be a payment that is not overwriting 'reason' that the contribution was originally created.

To replicate:
1. Set Default invoice payment page in CiviContribute Setting (eg contribution page ID = 4)
2. Add a pay later contribution using Contribution page(eg ID =2)
3. Click on Pay Now button for a pay later contribution under user dashboard and complete the transaction

Result:
civicrm_contribution.contribution_page_id is updated to 4 and also source.

To fix:
1. Don't change(update) civicrm_contribution.contribution_page_id
2. Update civicrm_contribution.source to left(concat($original_source, '; paid later via ', $new_source, ' (page ', $new_contrib_page_id, ')'), 255)

Note: please check other contribution fields if there are updated and report back here if any.