1) first of all, you will NOT be "switching to active duty". IF you go to basic training for GUARD or RESERVES (even if only for 'inactive status' - which i never heard of, why would the reserves pay to send you to basic only to not have you drill?) you will be considered 'prior service' by the active side of the house, and there is virtually ZERO opportunities for 'prior service'. if you want to go active, go active from day one, or you never will.
2 - IF you are on guard and reserves, you have NO tricare or insurance covererage at all - unless you are on duty, on orders, or unless you subscribe to tricare reserve select (AND pay premiums for it). you do not have to get TRS, but of course with obamacare, you do have to carry insurance of some kind and if MediCal meets the requirement, don't sweat it.
3 - if you do go active, you can purchase outside insurance if you want, but why would you WANT to? if you are on active duty tricare is free (no premiums) for you and your eligible dependents. you also have no deductible, no prescription costs, etc. a word of caution here: IF you are active duty and have tricare AND if you buy an outside insurance plan - you will be required to meet the deductible, co-pay, cost-shares, out-of-pocket maximums FIRST before tricare will pay ANYTHING at all. this is because BY LAW, tricare is the last payer on a claim. this is why it would make NO SENSE if on active duty to have an outside health insurance - it would only cost you more.
4 - if you are guard/reserves, look into TRS. IF you currently have to pay any co-pays, deductibles on MediCal (is that Title 19?) chances are TRS will save you money. premiums are very affordable for single people, deductible last i knew was $300 (much less than the 3 to 5 THOUSAND everyone else pays), and you can get prescriptions for free - filled at the local military facility or i think through express scrips