QB Issue Resolution:

LLC is not a business type

How you will be taxed as a new business is the key,  sole proprietor, partnership ??

see this for whether or not you are allowed to keep the EIN

https://www.irs.gov/businesses/small-businesses-self-employed/do-you-need-a-new-ein

Resolution for Issue 'Conversion from Corp to LLC' available: Yes (Solved).
Source: Intuit Community forum.

Conversion from Corp to LLC: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for conversion
Quickbooks Repair Pro will resolve it efficiently and affordably. Guaranteed resolution of your issue, or you pay nothing!

Error Recovery for conversion [Canada]
E-Tech is the #1 Quickbooks data migration and Conversion service provider.

Get 50% off Quickbooks Online

Click Here for 50% off!

Additional Support Options: