If Republican regain control of the US Senate next year and Grassley has to resign, Mitch McConnell become President Pro Tempore of the US Senate.

Pat Grassley gets appointed to the US Senate when ever Chuck leaves?

US Senator Patty Murray(D-WA) will be President Pro Tempore of the US Senate. Feinstein(D-CA) will not seek re-election or lose in the November 2024 General Election to a Democrat (Linda Sánchez-D).

Jill Krowinski(Speaker of House since 2021/member of House since 2013)
Becca Ballant(President Pro Temp of Senate since 2021/Member of Senate since 2015)
TJ Donovan(Attorney General since 2017) Donovan runs for Leahy’s seat in 2022/Molly Gray runs for Sander’s seat in 2024.

for the 2022 US Senate Race?

State Senate Minority Leader Randy Brock or
State House Minority Leader Patricia McCoy or
Scott Milne

37)HI(Schatz-D)
38)CA(Padilla-D)
39)NY(Schumer-D)
40)MD(Van Hollen-D)
41)CT(Blumenthal-D)
42)VT(Leahy-D or Welch-D)
43)OR(Wyden-D)
44)WA(Murray-D)
45)IL(Duckworth-D)
46)CO(Bennet-D)
47)NH(Hassan-D)
48)NV(Cortez Masto-D)
49)AZ(Kelly-D)
50)GA(Warnock-D)
51)PA(Fetterman-D)
52)WI(Barnes-D)

VT Attorney General T J Donovan is the perfect candidate to replace Leahy.

Donovan won 3 statewide elections by landslide margins.
US Representative Peter Welch has won 7 out of his 9 statewide elections by a landslide margins. The problem with Welch is he will be 75 years old in 2022. The maximum number of term Welch will serve is 2 term.
Lt Governor Molly Gray is in her first term. Gray is likely to replace Sanders in 2024.

Which of them are likely to leave in 2028 voluntarily and/or involuntarily?
Leahy(D-VT) will be 88 years in 2028, Might run for a 10th term.
Murray(D-WA) next in line to be US Senate President Pro Temp.after Leahy(VT) and Feinstein(CA) leave the US Senate.
Wyden(D-OR)
Schumer(D-NY)
These US Senators will be in the US Senate for at least 30 years.

Bennet(D-CO),Blumenthal(D-CT),Schatz(D-HI),Duckworth(D-IL),and Van Hollen(D-MD) will be in the US.Senate as long as they want.

Hassan(D-NH),Cortez Masto(D-NV),Kelly(D-AZ),Warnock(D-GA) and Fetterman(D-PA) if he gets elected in 2022 are highly vulnerable in 2028.

Safe D
37)VT(Leahy-D)
38)HI(Schatz-D)
39)NY(Schumer-D)
40)CA(Padilla-D)
41)MD(Van Hollen-D)
42)CT(Blumenthal-D)
43)OR(Wyden-D)
44)WA(Murray-D)
45)IL(Duckworth-D)
Likely D
46)CO(Bennet-D)
Lean D
47)NH(Hassan-D)
48)NV(Cortez Masto-D)
Tilt D
49)AZ(Kelly-D)
Tossup
50)PA(Fetterman-D)
51)GA(Warnock-D) will be decided in the 12/2022 or 1/2023 General Election runoff.
Tilt R
52)WI(Johnson-R vs Barnes-D) will be similar to 2010(Johnson-R vs Feingold-D) and 2016(Johnson-R vs Feingold-D)
Lean R
53)NC(Budd-R vs Beasley-D)
54)OH(Mandel-R vs Ryan-D)
Likely R
55)FL(Rubio-R vs Demings-D)

37)VT(Leahy-D or Welch-D)
38)HI(Schatz-D)
39)NY(Schumer-D)
40)CA(Padilla-D)
41)MD(Van Hollen-D)
42)CT(Blumenthal-D)
43)OR(Wyden-D)
44)WA(Murray-D)
45)IL(Duckworth-D)
46)CO(Bennet-D)
47)NH(Hassan-D)
48)NV(Cortez Masto-D)
49)AZ(Kelly-D)
50)GA(Warnock-D)
51)PA(Fetterman-D)

WI(Johnson-R vs Barnes-D) is a Tilt/Lean R state similar to the 2010 WI US Senate Race(Johnson-R vs Feingold-D) and 2016 WI US Senate Race(Johnson-R vs Feingold-D).