Use bad UI till overall redesign or add good UI in small steps?

I started working as a UI/UX Designer for a large company with a big website.

Since the developers are busy with developing backend functions, the resources are limited right now.

Should I start designing good UI even though they wont be able to change it globally (for example, forms, modals, etc.)?

Or should I design with the old, bad UI so the users feel consistency until we manage to make an overall redesign?