DegeReguard
You said “My company says in order to upgrade i must be available and not attend the scheduled drill..” is pretty damaging to your airline. Get that in writing by asking them to confirm this decision via email to a an appropriate company official (Chief Pilot, Training manager, etc responsible for this decision)
Remember this: USERRA protects your military service no matter how inconvenient for you OR your employer. Agree with Slim11 if able, get the two parties to agree in a mutually beneficial solution. However, your civilian Captain seniority based awarded position is protected the moment you return from military service. No matter how inconvenient it is for your employer. Your employer may try to suggest they cannot train you, wait etc. If they do, seriously consider filing a complaint with the DOL VETS office. Let your employer know that is your intent as well as your union Military committee if they have this group. This is fair to allow them to do the lawful thing and get you trained to your earned job protected by USERRA in the collectively known Escalator clause. If they continue to refuse, more detail for your complaint. Guessing company lawyers/HR folks will be seriously reviewing USERRA to ensure their position is legal and defensible. Airline pilots have it the easiest in most jobs regarding the Escalator clause, particularly if you have a seniority based system. Ensure your bid reflects a Captain position prior to military service, show DOL VETS on the complaint your class date before military service and your position is factually demonstrated.
Looking at the Employer Support of the Guard and Reserve (ESGR) FAQ’s that is designed to help Employers (Company) understand the USERRA Statute and their obligations to us as Employees.

Employer Support of the Guard and Reserve > USERRA > Frequently Asked Questions
What position is the employee entitled to upon reemployment?
“As a general rule, the employee is entitled to reemployment in the job position that he or she would have attained with reasonable certainty if not for the absence due to military service. This position is known as the escalator position. The principle behind the escalator position is that, if not for the period of military service, the employee could have been promoted (or, alternatively, demoted, transferred, or laid off) due to intervening events. The escalator principle requires that the employee be reemployed in a position that reflects with reasonable certainty the pay, benefits, seniority, and other job perks that he or she would have attained if not for the period of service.”

I’m guessing you’re in a union and have a seniority based system, so as said above, we have an easy way to prove the “escalator” position and so does the employer. I recommend always leaving your bids on file depending on contract to ensure your airline knows your “escalator” choices before military service to remove any misunderstanding. In your case, it was already determined. Airline cannot deny your Captain status upon return.
Here is another ESGR FAQ for your airline:

Does the reemployment position include elements such as seniority, status, and rate of pay?

“Yes. The reemployment position includes the seniority, status, and rate of pay that an employee would ordinarily have attained in that position given his or her job history, including prospects for future earnings and advancement. The employer must determine the seniority rights, status, and rate of pay as though the employee had been continuously employed during the period of service.
The seniority rights, status, and pay of an employment position include those established (or changed) by a collective bargaining agreement, employer policy, or employment practice. In particular, the employee’s status in the reemployment position could include opportunities for advancement, general working conditions, job location, shift assignment, rank, responsibility, and geographical location. If an opportunity for promotion, or eligibility for promotion that the employee missed during service is based on a skills test or examination, then the employer should give him or her a reasonable amount of time to adjust to the employment position and then give a skills test or examination.”

IMO, yours is an easy circumstance. Kudos trying to make both military and airline happy, but you are the protected party in this three way relationship of shared employee and soldier. Train with your Army unit if you must, your airline has little to fall back denying you your upgrade (unless your displaced through a furlough or cutbacks that apply to others of your seniority) PM me like Slim11 offered if desire