Considerations in negotiating pay, shift differentials, accountability

How/where do I think about pay, shift differential, accountability/responsibilities?

Company is expanding quickly. Corporate visit next week. In fact, Director of HR hijacked a meeting on Friday to ask how/if full-time would work for me. I am currently a part-time, weekends pharmacist. However, I log extra hours as I am completing licensing in other states.

I was hired for Saturday and Sundays, 7am-330pm. However, they opened for about four Saturdays now. No Sundays yet. (Growing pains - team not big enough, etc). I started four months ago. So I have been working on weekdays to expose myself to different tasks.

From what I can tell, day shift pharmacists ONLY check prescriptions. They verify the product. However, the weekends do not have enough staff. I literally do everything like a mini manager. I am cross trained in everything. There's constant staff training. I walk around the facility driving production because there's no team leads around.

Questions that I am pondering and need advice/insight/suggestions:

• Is it fair and/or reasonable to ask for same salary when transitioning from part-time to full-time status? The regular, day shift job posting at the moment offers 15% less. I was hired with a 'premium' because it's difficult to fill a permanent weekends role. A 40-hour work week would be 2/5 weekend hours.
• Should I offer to be MORE available on weekends? It would increase their potential for production and justify my current rate. (From Saturday/Sunday 7am-330pm to 7am-7pm)