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GP 14: Migrate BOB pool to USDC on Polygon

Proposal objective

This proposal is to replaces the underlying token from BOB to USDC for the zkBob pool on Polygon as discussed with the community.
The migration will replace the zkBob pool implementation to the new contract 0x5e93FA8819Bc074C717a92240a1689D1B81aAFcd and the Direct Deposit Queue implementation to 0x22299414E9e0E7663f8977a7EA6c1908F2167181. To allow further swaps of USDC to MATIC as part of the withdrawal process the Token Seller contract will be replaced by 0xDb53D01947252A5bEb488991971D8436AA2828A5.
All BOB owned by the zkBob pool contract will be swapped to USDC using BobSwap. The amount of BOB tokens paid as fees on BobSwap will be reimbursed with the corresponding amount of USDC from the Governance Safe account. The logic of the tokens swap is defined in the migrationToUSDC() method of the pool.

Proposal breakdown

To avoid undesirable interactions with the zkBob pool before the migration the proposal consists of two steps:
Step 1: Disable direct deposits
Transaction #32 in the Safe on Polygon contains two three setLimits calls to disallow users from transfering tokens directly to the pool by setting limits for tier 0, 1 and 254 to zero for direct deposits. Interim limits are set to the following values:
Tier
Single deposit
Daily user deposit
Daily deposit
Daily withdrawal
Pool size
Single DD
Default, id 0
10,000 BOB
10,000 BOB
300,000 BOB
300,000 BOB
2,000,000 BOB
0 BOB
KYC, id 254
20,000 BOB
20,000 BOB
300,000 BOB
300,000 BOB
2,000,000 BOB
0 BOB
Partners, id 1
100,000 BOB
100,000 BOB
300,000 BOB
300,000 BOB
2,000,000 BOB
0 BOB
Step 1 is executed 15 minutes prior to step 2.
Step 2: Migrate the pool from BOB to USDC
Transaction #33 in the Safe executes the following calls:
Tier
Single deposit
Daily user deposit
Daily deposit
Daily withdrawal
Pool size
Single DD
Daily user DD
Default, id 0
10,000 USDC
10,000 USDC
300,000 USDC
300,000 USDC
2,000,000 USDC
1,000 USDC
10,000 USDC
KYC, id 254
20,000 USDC
20,000 USDC
300,000 USDC
300,000 USDC
2,000,000 USDC
1,000 USDC
10,000 USDC
Partners, id 1
100,000 USDC
100,000 USDC
300,000 USDC
300,000 USDC
2,000,000 USDC
1,000 USDC
10,000 USDC

Additional verification

​The migration procedure was verified using the following Foundry-script.
​Another script verifies the correctness of the Safe transactions execution. The script could be placed to script/scripts directory of the zkBob contracts repo. The command to run the script is forge script -vvv -rpc-url <https://rpc.ankr.com/polygon> script/scripts/BOBPoolMigrationVerification.s.sol