The problem has been solved by adding a condition to help checking the upgrade progress and initiating upgrade membership.
==================================================================================
	public void upgradeMembership(Member member) {
		if (checkMembership(member)) {
			while (checkRemainingProgress(member) < 0) {
				if (member.getMembership() instanceof PlatinumMembership) {
					break;
				} else {
					member.setMembership(member.getMembership().upgradeMembership());
				}

			}
		}
	}
==================================================================================
Please input your Worker ID for logging in the system...
1
Please input your Member ID for logging in the system...
123
---Welcome to Morx Supermarket---
---To continue, please proceed your actions---
Input (1) for accessing Inventory System
Input (2) for accessing Checkout System
Input (3) for accessing Sales System
Input (4) for accessing Membership System
Input (5) for switching member
Input (6) to exit
2
---Welcome to Checkout System, aiden.---
Input commands for further management:
Input (1) for Checkout
Input (2) for refund
Input (3) for exit
1
No                  Description         Quantity            Price($)            
1                   candies             10                  300.0               
2                   Salt                60                  1200.0              
Please input (0) for proceed to checkout
Please input (1) for remove products
Please input (2) for modify quantities of products
Please input (3) to cancel the checkout process
2
Please input which number of products customers would like to modify from their cart.
1
Please input the quantity that they would like to change.
200
Task Completed.
Do you have any other actions to continue ?
Please input (0) to exit
Please input (1) to continue
0
No                  Description         Quantity            Price($)            
1                   candies             200                 6000.0              
2                   Salt                60                  1200.0              
Please input (0) for proceed to checkout
Please input (1) for remove products
Please input (2) for modify quantities of products
Please input (3) to cancel the checkout process
0
The total amount is: $6840.00
Please Input the amount of dollars that the customers paid
7000
Total changes is $160.00
The Order Reference Number is 2021-11-297
Input commands for further management:
Input (1) for Checkout
Input (2) for refund
Input (3) for exit
3
Exiting...
---To continue, please proceed your actions---
Input (1) for accessing Inventory System
Input (2) for accessing Checkout System
Input (3) for accessing Sales System
Input (4) for accessing Membership System
Input (5) for switching member
Input (6) to exit
4
---Hi aiden, To check aero's membership details, please proceed your actions---
---Input commands for further management---
Input (1) for checking the Customer Membership Status
Input (2) for checking the Customer's points
Input (3) for checking the status to another level of Membership
Input (4) for exit
1
The customer owns a Gold Membership.
---Hi aiden, To check aero's membership details, please proceed your actions---
---Input commands for further management---
Input (1) for checking the Customer Membership Status
Input (2) for checking the Customer's points
Input (3) for checking the status to another level of Membership
Input (4) for exit