Germany retaining two Derfflinger battlecruisers

How much would Germany do to keep two retained Derfflinger-class battlecruisers useful?

Substantial work is expected, alongside close international scrutiny that would complicate the treaty system.

The scenario retains two Derfflingers for use against major Baltic warships. Keeping them useful would require considerable work. Other powers would watch the ships closely, complicating the naval treaty system. No detailed modernisation scheme is specified.