Thank you for clarifying. In this case, a transfer cum continuity letter on the letter head of the new entity can be provided, requesting an acknowledgment from the employees.
I also suggest to have
• a communication about it through leadership to all in town hall, monthly all hands or others, followed by HR facilitated 1:1 query response session with employees.
• a common email to transferred employees on continued benefits and if any major thing changes.
• faqs can be created based on the pool of questions asked/received, circulated among transferred employees and stay employees, respectively.
These may be done prior to the letter handover. After fortnight, an open hour may be kept for transferred employees to ask questions in 1:1 either with HR or Leadership.