As someone who's primary job is to process conversions (money only, not addressing plan provisions or payroll), I can tell you that an in-kind transfer is much more difficult than a liquidation and wire. To speak to the comments above, if the money is insurance company separate accounts or omnibus accounts, an in-kind transfer is not possible. You would really only be able to move from one custodian to another where the funds are invested at the plan level. Arranging for an in-kind transfer as of a specific date takes a lot of coordination and agreement between the custodians, recordkeepers and the funds themselves and all must agree to the same schedule, and there will be lots of paperwork involved.
Do you have a copy of the conversion records that Guideline originally sent over? You may want to look at those records to see for yourself if they had the information necessary. For example, did the records include the full name, Social Security Number and amount by source and fund by participant that was transferred? Also, did Guideline provide a report by participant with a beginning balance for the year, contributions, distributions, earnings, fees, loan data, and transfer out, etc.? Did they provide any census data? Did you map over the funds, or did they invest as per new enrollment allocations (or QDIA)? Mapping adds another layer of complexity, especially if fund data was not provided at the participant level.
There are some recordkeepers who use their own identifiers and don't provide Social Security Numbers, which can make the conversion process difficult. Also, receiving the data for the current year doesn't always happen. I often have to make several requests to get current year data or end up getting several files with pieces of data and have to try to rebuild the year with what data I did receive (which I prefer NOT to do).
If you ever do a conversion again, I would ask for sample copies of the records that are going to be sent over and ask to be cc:d in any communications between the recordkeepers so you can see what information is and is not being provided and when. Ultimately, if you are the Plan Fiduciary, all responsibility lands on you, so being very involved it the conversion process would be advisable.