There are a few ways to move your Lucidchart diagrams over to diagrams.net. Although diagrams.net can save the diagrams to a few different locations, Google Drive is the recommended location. None of these methods are perfect. You may have to do some editing after the move like fixing the text or reinserting some images.
Copy and paste method - https://www.diagrams.net/doc/faq/lucidchart-import
VSDX/VDX method - export the diagram in Lucidchart to VSDX or VDX format then open in diagrams.net
Diagrams.net and draw.io Importer method
Install extension from https://chrome.google.com/webstore/detail/diagramsnet-and-drawio-im/cnoplimhpndhhhnmoigbanpjeghjpohi
Open/login to Lucidchart in Google Chrome
Open another tab in Chrome and go to https://app.diagrams.net/tools/lucid/import.html
Select the diagrams to import and click ‘Import’ (some filenames may be duplicated)
Open the downloaded zip file and extract the files (the files will have an extension .drawio, for example filename.drawio)
Copy the drawio files to Google Drive
The drawio files may be associated with Google Docs since they are just XML files
To open them, right-click on the file and select ‘Open with’-> diagrams.net
