If your full‐time job is suddenly turned into a part‐time job you are, for all intents and purposes, being fired. No employer can realistically expect someone who relied on a full‐time income to suddenly be able to cope with a part‐time income. And no employer can assume a suddenly part‐time employee will be able to find a second part‐time job that fits their now altered schedule. The employer is trying to have its cake and eat it too: eliminate a full‐time position but keep the person who has been doing the job well for as long as possible. There's no point in trying to argue them out of their decision. If they desperately wanted to keep you, they would have fired someone else and moved you into their place. And if there was some way to expand your job's responsibilities in order to keep it full‐time they would have just added to your workload. They've reached a decision they're not going to change. Rather than debating, respond in a similar manner to their wanting to have the best of both worlds. Explain that this is, in effect, a termination, so ask for a severance package. Then, add that you would be happy to fill in as a part‐timer for as long as they need you up until you land a new full‐time position elsewhere. Your part‐time pay, however long it lasts, will be a good supplement to your severance. Since they knew you'd only probably remain with them long enough to find a new job, your being a temporary part‐timer won't be a surprise. And they had to be prepared for a severance request from the moment they conceived the plan. You're just suggesting they do both.
This script can be modified to: